diff --git a/.gitignore b/.gitignore
old mode 100644
new mode 100755
diff --git a/README.md b/README.md
old mode 100644
new mode 100755
index dd42196..28486a9
--- a/README.md
+++ b/README.md
@@ -1,9 +1,9 @@
 # Overview
-1. This is my Python (2.7) Leetcode solution.
+1. This is my Python Leetcode solution.
 As time grows, this also become a guide to prepare for software engineer interview.
 
-2. The solution is at `problems/the-file-name/`.
-For example, `merge-sorted-array.py`'s solution is at `https://leetcode.com/problems/merge-sorted-array/`.
+1. The solution is at `problems/python/` or `problems/python3/`.
+For example, `merge-sorted-array.py`'s solution is at `https://leetcode.com/problems/python/merge-sorted-array/`.
 
 2. I really take time tried to make the best solution and collect the best resource that I found.  
 Because I wanted to help others like me.  
@@ -13,6 +13,7 @@ Please [BUY ME A COFFEE](https://www.buymeacoffee.com/chriswu) if you want to sh
 # Leetcode Problem Lists
 I found it makes sense to solve similar problems together, so that we can recognize the problem faster when we encounter a new one. My suggestion is to skip the HARD problems when you first go through these list.
 
+* https://neetcode.io/practice (150 problems with video explaination)
 * https://www.programcreek.com/2013/08/leetcode-problem-classification/
 * https://github.com/wisdompeak/LeetCode
 * https://docs.google.com/spreadsheets/d/1SbpY-04Cz8EWw3A_LBUmDEXKUMO31DBjfeMoA0dlfIA/edit#gid=126913158 ([huahua](https://www.youtube.com/user/xxfflower/videos)).
@@ -37,7 +38,7 @@ I found it makes sense to solve similar problems together, so that we can recogn
 3. [What should I know from the CLRS 3rd edition book if my aim is to get into Google?](https://www.quora.com/What-should-I-know-from-the-CLRS-3rd-edition-book-if-my-aim-is-to-get-into-Google/answer/Jimmy-Saade)
 
 ## Data Structures and Algorithms for beginners
-If you are new or know nothing about data structures and algorithms, I recommend [this course](<https://classroom.udacity.com/courses/ud513>). This course is taught in Python and design to help you find job and do well in the interview.
+If you are new or know nothing about data structures and algorithms, I recommend [this course](<https://www.udacity.com/course/data-structures-and-algorithms-in-python--ud513>). This course is taught in Python and design to help you find job and do well in the interview.
 
 
 # System Design
diff --git "a/common/bellman\342\200\223ford.py" "b/common/bellman\342\200\223ford.py"
old mode 100644
new mode 100755
diff --git a/common/binary-search-tree.py b/common/binary-search-tree.py
old mode 100644
new mode 100755
diff --git a/common/bst-in-order.py b/common/bst-in-order.py
old mode 100644
new mode 100755
diff --git a/common/dijkstra.py b/common/dijkstra.py
old mode 100644
new mode 100755
diff --git a/common/heap-sort.py b/common/heap-sort.py
old mode 100644
new mode 100755
index eb2e11c..3d56e39
--- a/common/heap-sort.py
+++ b/common/heap-sort.py
@@ -1,35 +1,33 @@
+#O(LogN), used when part of the array is already heapified.
+def heapify(A, N, i):
+	largest = i
+	l = i*2+1
+	r = i*2+2
+
+	if l<N and A[l]>A[largest]: largest = l
+	if r<N and A[r]>A[largest]: largest = r
+	if largest!=i:
+		A[largest], A[i] = A[i], A[largest]
+		heapify(A, N, largest)
+
+#O(NLogN)
 def heapSort(A):
-	#build max heap
-	def heapify(A, n, i):
-		if i>=n: return
-		l, r = i*2+1, i*2+2
-		left = A[l] if l<n else float('-inf')
-		right = A[r] if r<n else float('-inf')
-
-		if left>A[i] or right>A[i]:
-			if left>right:
-				A[i], A[l] = A[l], A[i]
-				heapify(A, n, l)
-			else:
-				A[i], A[r] = A[r], A[i]
-				heapify(A, n, r)
-
-	n = len(A)
-
-	for i in reversed(xrange(n)):
-		heapify(A, n, i)
-
-	for i in reversed(xrange(1, n)):
-		A[i], A[0] = A[0], A[i]
+	N = len(A)
+
+	#build max heap, O(NLogN). Can be optimized to the O(N).
+	for i in range(N//2-1, -1, -1):
+		heapify(A, N, i)
+	
+	#keep swapping the largest
+	for i in range(N-1, -1, -1):
+		A[0], A[i] = A[i], A[0]
 		heapify(A, i, 0)
 
 
-
-A = [21, 4, 1, 3, 9, 20, 25, 6, 21, 14]
+A = [12, 11, 13, 5, 6, 7]
 heapSort(A)
-print A
+print(A)
 
-"""
-Time Complexity O(NLogN) in best, average, worst case.
-Space Complexity O(1)
-"""
\ No newline at end of file
+A = [1, 3, 5, 4, 6, 13, 10, 9, 8, 15, 17]
+heapSort(A)
+print(A)
\ No newline at end of file
diff --git a/common/insertion-sort.py b/common/insertion-sort.py
old mode 100644
new mode 100755
diff --git a/common/knapsack.py b/common/knapsack.py
old mode 100644
new mode 100755
diff --git a/common/merge-sort-in-place.py b/common/merge-sort-in-place.py
old mode 100644
new mode 100755
diff --git a/common/merge-sort.py b/common/merge-sort.py
old mode 100644
new mode 100755
diff --git a/common/min-heap.py b/common/min-heap.py
old mode 100644
new mode 100755
diff --git a/common/quick-sort.py b/common/quick-sort.py
old mode 100644
new mode 100755
diff --git a/common/radix-sort.py b/common/radix-sort.py
old mode 100644
new mode 100755
diff --git a/common/trie.py b/common/trie.py
old mode 100644
new mode 100755
diff --git a/problems/accounts-merge.py b/problems/accounts-merge.py
deleted file mode 100644
index 30e4787..0000000
--- a/problems/accounts-merge.py
+++ /dev/null
@@ -1,52 +0,0 @@
-from collections import defaultdict
-
-class Solution(object):
-    def accountsMerge(self, accounts):
-        graph = defaultdict(list)
-        merged = set()
-        ans = []
-
-        #[0]
-        for data in accounts:
-            emails = data[1:]
-            for i, email in enumerate(emails):
-                graph[email].extend(emails[:i])
-                graph[email].extend(emails[i+1:])
-        
-        for data in accounts:
-            name = data[0]
-            visited = set()
-            stack = [data[1]] #[2]
-
-            if data[1] in merged: continue #[1]
-            
-            while stack:
-                e = stack.pop()
-                if e in visited: continue
-                visited.add(e)
-                stack.extend(graph[e])
-            
-            merged.update(visited)
-            ans.append([name]+sorted(list(visited))) #[3]
-        
-        return ans
-
-"""
-Treat each email as a node.
-Build an adjacency graph. [0]
-
-For every account's data
-First, lets check if the first email is already merged. [1]
-If the first email is already merged to other groups, then other emails will be in another group as well.
-So don't need to check.
-
-Second, do a DFS starting from the first email. Put all the connected nodes into visited. [2]
-And append the sorted(visited) to the ans with name. [3]
-
-Let N be the total number of emails. M be the total number of final groups.
-Build the graph takes O(N).
-For each final groups, we make a DFS to all nodes, taking O(MN).
-Sort the group takes O((N/M)*Log(N/M)) -> O((N/M)*(logN-LogM))
-Time: O(MN)
-Space: O(N)
-"""
\ No newline at end of file
diff --git a/problems/find-and-replace-in-string.py b/problems/find-and-replace-in-string.py
deleted file mode 100644
index ed407d1..0000000
--- a/problems/find-and-replace-in-string.py
+++ /dev/null
@@ -1,23 +0,0 @@
-"""
-`p` is the index on s which already processed.
-"""
-class Solution(object):
-    def findReplaceString(self, s, indices, sources, targets):
-        p = -1
-        ans = ''
-        
-        memo = {}
-        for i, index in enumerate(indices):
-            memo[index] = (sources[i], targets[i])
-        
-        for i in xrange(len(s)):
-            if i<=p: continue
-            
-            if i in memo and (s[i:i+len(memo[i][0])] if i+len(memo[i][0])<len(s) else s[i:])==memo[i][0]:
-                ans += memo[i][1]
-                p = i+len(memo[i][0])-1
-            else:
-                ans += s[i]
-                p = i
-            
-        return ans
\ No newline at end of file
diff --git a/problems/k-closest-points-to-origin.py b/problems/k-closest-points-to-origin.py
deleted file mode 100644
index 8af9889..0000000
--- a/problems/k-closest-points-to-origin.py
+++ /dev/null
@@ -1,15 +0,0 @@
-class Solution(object):
-    def kClosest(self, points, k):
-        h = []
-        
-        for x, y in points:
-            d = (x**2+y**2)**0.5
-            if len(h)>=k and -h[0][0]>d:
-                heapq.heappop(h)
-                heapq.heappush(h, (-d, x, y))
-            elif len(h)>=k and -h[0][0]<=d:
-                pass
-            else:
-                heapq.heappush(h, (-d, x, y))
-        
-        return [(x, y) for _, x, y in h]
\ No newline at end of file
diff --git a/problems/maximal-square.py b/problems/maximal-square.py
deleted file mode 100644
index 4ba17d6..0000000
--- a/problems/maximal-square.py
+++ /dev/null
@@ -1,13 +0,0 @@
-class Solution(object):
-    def maximalSquare(self, grid):
-        if not grid or not grid[0]: return 
-        M, N = len(grid), len(grid[0])
-        
-        dp = [[0 for _ in xrange(N+1)] for _ in xrange(M+1)]
-        ans = 0
-        for i in xrange(M):
-            for j in xrange(N):
-                if grid[i][j]=='1':
-                    dp[i+1][j+1] = min(dp[i][j], dp[i][j+1], dp[i+1][j])+1
-                    ans = max(ans, dp[i+1][j+1])
-        return ans**2
\ No newline at end of file
diff --git a/problems/next-permutation.py b/problems/next-permutation.py
deleted file mode 100644
index f228cc4..0000000
--- a/problems/next-permutation.py
+++ /dev/null
@@ -1,32 +0,0 @@
-"""
-This answer is the python version of the offical answer.
-
-Time: O(N)
-Space: O(1)
-"""
-class Solution(object):
-    def nextPermutation(self, nums):
-        def reverse(start):
-            end = len(nums)-1
-            
-            while start<end:
-                swap(start, end)
-                start += 1
-                end -= 1
-        
-        def swap(i, j):
-            nums[i], nums[j] = nums[j], nums[i]
-        
-        
-        i = len(nums)-2
-        
-        while i>=0 and nums[i+1]<=nums[i]:
-            i -= 1
-        
-        if i>=0:
-            j = len(nums)-1
-            while nums[j]<=nums[i]: j -= 1
-            swap(i, j)
-        
-        reverse(i+1)
-        return nums
\ No newline at end of file
diff --git a/problems/profitable-schemes.py b/problems/profitable-schemes.py
deleted file mode 100644
index ead8e92..0000000
--- a/problems/profitable-schemes.py
+++ /dev/null
@@ -1,19 +0,0 @@
-"""
-TLE
-dp[i][n][p] := considering profit[:i], what is the number of ways produce profit p with n people.
-"""
-class Solution(object):
-    def profitableSchemes(self, maxMember, minProfit, group, profit):
-        P = sum(profit)
-        N = sum(group)
-        dp = [[[0 for _ in xrange(P+1)] for _ in xrange(N+1)] for _ in xrange(len(profit)+1)]
-        dp[0][0][0] = 1
-        
-        count = 0
-        for i in xrange(1, len(profit)+1):
-            for n in xrange(N+1):
-                for p in xrange(P+1):
-                    dp[i][n][p] = (dp[i-1][n-group[i-1]][p-profit[i-1]] if p-profit[i-1]>=0 and n-group[i-1]>=0 else 0) + dp[i-1][n][p]
-                    if i==len(profit) and p>=minProfit and n<=maxMember: count += dp[i][n][p]
-        return count
-        
\ No newline at end of file
diff --git a/problems/01-matrix.py b/problems/python/01-matrix.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/01-matrix.py
rename to problems/python/01-matrix.py
diff --git a/problems/3sum-closest.py b/problems/python/3sum-closest.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/3sum-closest.py
rename to problems/python/3sum-closest.py
diff --git a/problems/3sum.py b/problems/python/3sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/3sum.py
rename to problems/python/3sum.py
diff --git a/problems/4sum.py b/problems/python/4sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/4sum.py
rename to problems/python/4sum.py
diff --git a/problems/python/accounts-merge.py b/problems/python/accounts-merge.py
new file mode 100755
index 0000000..91e03ef
--- /dev/null
+++ b/problems/python/accounts-merge.py
@@ -0,0 +1,134 @@
+"""
+Treat each email as a node.
+Build an adjacency graph. [0]
+
+For every account's data
+First, lets check if the first email is already merged. [1]
+If the first email is already merged to other groups, then other emails will be in another group as well.
+So don't need to check.
+
+Second, do a DFS starting from the first email. Put all the connected nodes into visited. [2]
+And append the sorted(visited) to the ans with name. [3]
+
+Let N be the total number of emails. M be the total number of final groups.
+Build the graph takes O(N).
+For each final groups, we make a DFS to all nodes, taking O(MN).
+Sort the group takes O((N/M)*Log(N/M)) -> O((N/M)*(logN-LogM))
+Time: O(MN)
+Space: O(N)
+"""
+from collections import defaultdict
+
+class Solution(object):
+    def accountsMerge(self, accounts):
+        graph = defaultdict(list)
+        merged = set()
+        ans = []
+
+        #[0]
+        for data in accounts:
+            emails = data[1:]
+            for i, email in enumerate(emails):
+                graph[email].extend(emails[:i])
+                graph[email].extend(emails[i+1:])
+        
+        for data in accounts:
+            name = data[0]
+            visited = set()
+            stack = [data[1]] #[2]
+
+            if data[1] in merged: continue #[1]
+            
+            while stack:
+                e = stack.pop()
+                if e in visited: continue
+                visited.add(e)
+                stack.extend(graph[e])
+            
+            merged.update(visited)
+            ans.append([name]+sorted(list(visited))) #[3]
+        
+        return ans
+
+
+#Union Find
+class Solution(object):
+    def accountsMerge(self, accounts):
+        def find(x):
+            p = parents[x]
+            while p!=parents[p]:
+                p = find(p)
+            parents[x] = p
+            return p
+        
+        def union(x, y):
+            p1, p2 = find(x), find(y)
+            if p1==p2: return
+            parents[p2] = p1
+        
+        parents = {}
+        mailToName = {}
+        
+        for account in accounts:
+            name = account[0]
+            root = account[1]
+            if root not in parents: parents[root] = root
+            root = find(root)
+            mailToName[root] = name
+            
+            for i in xrange(2, len(account)):
+                email = account[i]
+                if email in parents:
+                    union(parents[email], root)
+                    root = find(root)
+                parents[email] = root
+        
+        rootToMails = collections.defaultdict(list)
+        for email in parents:
+            rootToMails[find(email)].append(email)
+        
+        ans = []
+        for root in rootToMails:
+            name = mailToName[root]
+            mails = rootToMails[root]
+            ans.append([name]+sorted(mails))
+        
+        return ans
+
+#DFS
+class Solution(object):
+    def accountsMerge(self, accounts):
+        
+        #build adjacency list
+        adj = collections.defaultdict(list)
+        for account in accounts:
+            name = account[0]
+            email0 = account[1]
+            for i in xrange(2, len(account)):
+                email = account[i]
+                adj[email0].append(email)
+                adj[email].append(email0)
+        
+        #iterate accounts and dfs each email group
+        ans = []
+        visited = set() #store all the visited email
+        for account in accounts:
+            name = account[0]
+            email0 = account[1]
+            if email0 in visited: continue
+            
+            #dfs
+            group = set() #store the email group related to email0
+            stack = [email0]
+            while stack:
+                email = stack.pop()
+                if email in group or email in visited: continue
+                group.add(email)
+                visited.add(email)
+                for nei in adj[email]:
+                    stack.append(nei)
+            
+            ans.append([name]+sorted(list(group)))
+        
+        return ans
+            
\ No newline at end of file
diff --git a/problems/add-binary.py b/problems/python/add-binary.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/add-binary.py
rename to problems/python/add-binary.py
diff --git a/problems/add-digits.py b/problems/python/add-digits.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/add-digits.py
rename to problems/python/add-digits.py
diff --git a/problems/python/add-strings.py b/problems/python/add-strings.py
new file mode 100755
index 0000000..9cdd7f9
--- /dev/null
+++ b/problems/python/add-strings.py
@@ -0,0 +1,34 @@
+class Solution(object):
+    def addStrings(self, nums1, nums2):
+        ans = ''
+        i = len(nums1)-1
+        j = len(nums2)-1
+        
+        carry = 0
+        while 0<=i and 0<=j:
+            n1 = int(nums1[i])
+            n2 = int(nums2[j])
+            total = n1+n2+carry
+            n = total%10
+            carry = 1 if total>=10 else 0
+            ans = str(n)+ans
+            i -= 1
+            j -= 1
+        
+        while 0<=i:
+            total = int(nums1[i])+carry
+            n = total%10
+            carry = 1 if total>=10 else 0
+            ans = str(n)+ans
+            i -= 1
+        
+        while 0<=j:
+            total = int(nums2[j])+carry
+            n = total%10
+            carry = 1 if total>=10 else 0
+            ans = str(n)+ans
+            j -= 1
+        
+        if carry: ans = str(carry)+ans
+        
+        return ans
\ No newline at end of file
diff --git a/problems/add-two-numbers-ii.py b/problems/python/add-two-numbers-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/add-two-numbers-ii.py
rename to problems/python/add-two-numbers-ii.py
diff --git a/problems/add-two-numbers.py b/problems/python/add-two-numbers.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/add-two-numbers.py
rename to problems/python/add-two-numbers.py
diff --git a/problems/alien-dictionary.py b/problems/python/alien-dictionary.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/alien-dictionary.py
rename to problems/python/alien-dictionary.py
diff --git a/problems/all-nodes-distance-k-in-binary-tree.py b/problems/python/all-nodes-distance-k-in-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/all-nodes-distance-k-in-binary-tree.py
rename to problems/python/all-nodes-distance-k-in-binary-tree.py
diff --git a/problems/python/amount-of-new-area-painted-each-day.py b/problems/python/amount-of-new-area-painted-each-day.py
new file mode 100755
index 0000000..a5504d5
--- /dev/null
+++ b/problems/python/amount-of-new-area-painted-each-day.py
@@ -0,0 +1,40 @@
+"""
+1. Build sorted records = [(position, index, isStart)...]
+2. Iterate through all positions and maintain a box with all the "index" of the records its position is in start ~ end
+3. The smallest index in the box is the actual one that is paiting.
+
+Time: O(NLogN+P), N is the count of paint. Sorting the records takes NLogN. P is the max position.
+Although there is a while loop when iterate through P, each record is only being iterated once.
+O(NLogN + P + NLogN) ~= O(NLogN + P)
+Space: O(N)
+"""
+from sortedcontainers import SortedList
+
+class Solution(object):
+    def amountPainted(self, paint):
+        ans = [0]*len(paint)
+        box = SortedList()
+        records = []
+        maxPos = float('-inf')
+        
+        #[1]
+        for i, (start, end) in enumerate(paint):
+            records.append((start, i, -1))
+            records.append((end, i, 1))
+            maxPos = max(maxPos, end)
+        
+        records.sort()
+        
+        #[2]
+        i = 0
+        for pos in xrange(maxPos+1):
+            while i<len(records) and records[i][0]==pos:
+                _, index, t = records[i]
+                if t==-1:
+                    box.add(index)
+                else:
+                    box.remove(index)
+                i += 1
+            
+            if box: ans[box[0]] += 1 #[3]
+        return ans
\ No newline at end of file
diff --git a/problems/analyze-user-website-visit-pattern.py b/problems/python/analyze-user-website-visit-pattern.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/analyze-user-website-visit-pattern.py
rename to problems/python/analyze-user-website-visit-pattern.py
diff --git a/problems/backspace-string-compare.py b/problems/python/backspace-string-compare.py
old mode 100644
new mode 100755
similarity index 81%
rename from problems/backspace-string-compare.py
rename to problems/python/backspace-string-compare.py
index f3df6d2..cec9e8a
--- a/problems/backspace-string-compare.py
+++ b/problems/python/backspace-string-compare.py
@@ -82,4 +82,24 @@ def backspaceCompare(self, s1, s2):
             j -= 1
             
         return True
-        
\ No newline at end of file
+
+
+class Solution(object):
+    def backspaceCompare(self, s, t):
+        def helper(S):
+            ans = ''
+            i = len(S)-1
+            backspaceCount = 0
+            
+            while i>=0:
+                if S[i]=='#':
+                    backspaceCount += 1
+                else:
+                    if backspaceCount>0:
+                        backspaceCount -= 1
+                    else:
+                        ans += S[i]
+                i -= 1
+            return ans
+        
+        return helper(s)==helper(t)
\ No newline at end of file
diff --git a/problems/python/balance-a-binary-search-tree.py b/problems/python/balance-a-binary-search-tree.py
new file mode 100755
index 0000000..015934d
--- /dev/null
+++ b/problems/python/balance-a-binary-search-tree.py
@@ -0,0 +1,36 @@
+"""
+Imagine a list of sorted nodes, if we wanted to use the nodes to form a balanced BST, which root should we use?
+Yes, the one in the middle, since it can evenly spread two groups of nodes. This is what `getRoot()` does.
+And we do the same for the left half and right half. And so on. And so on...
+
+Time: O(N), N is the number of nodes.
+Space: O(N)
+"""
+class Solution(object):
+    def balanceBST(self, root):
+        def getInorderNodes(root):
+            nodes = []
+            stack = []
+            node = root
+            
+            while node or stack:
+                while node:
+                    stack.append(node)
+                    node = node.left
+                
+                node = stack.pop()
+                nodes.append(node)
+                node = node.right
+            
+            return nodes
+                
+        def getRoot(l, r):
+            if l>r: return None
+            m = (l+r)/2
+            root = inorderNodes[m]
+            root.left = getRoot(l, m-1)
+            root.right = getRoot(m+1, r)
+            return root
+            
+        inorderNodes = getInorderNodes(root)
+        return getRoot(0, len(inorderNodes)-1)
\ No newline at end of file
diff --git a/problems/balanced-binary-tree.py b/problems/python/balanced-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/balanced-binary-tree.py
rename to problems/python/balanced-binary-tree.py
diff --git a/problems/python/basic-calculator-ii.py b/problems/python/basic-calculator-ii.py
new file mode 100755
index 0000000..90ab539
--- /dev/null
+++ b/problems/python/basic-calculator-ii.py
@@ -0,0 +1,31 @@
+class Solution(object):
+    def calculate(self, s):
+        s += '+' #edge case, for last operation to be executed.
+        lastOperation = '+' #edge case, for the first currNum
+        operations = set(['+', '-', '*', '/'])
+        
+        stack = []
+        currNum = 0
+
+        for c in s:
+            if c.isdigit():
+                currNum = currNum*10 + int(c)
+            elif c in operations:
+                if lastOperation=='+':
+                    stack.append(currNum)
+                    currNum = 0
+                elif lastOperation=='-':
+                    stack.append(-currNum)
+                    currNum = 0
+                elif lastOperation=='*':
+                    currNum = stack.pop() * currNum
+                    stack.append(currNum)
+                    currNum = 0
+                elif lastOperation=='/':
+                    currNum = stack.pop() / currNum
+                    if currNum<0: currNum += 1
+                    stack.append(currNum)
+                    currNum = 0
+                lastOperation = c
+        
+        return sum(stack)
\ No newline at end of file
diff --git a/problems/best-time-to-buy-an-stock.py b/problems/python/best-time-to-buy-an-stock.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/best-time-to-buy-an-stock.py
rename to problems/python/best-time-to-buy-an-stock.py
diff --git a/problems/best-time-to-buy-and-sell-stock-iii.py b/problems/python/best-time-to-buy-and-sell-stock-iii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/best-time-to-buy-and-sell-stock-iii.py
rename to problems/python/best-time-to-buy-and-sell-stock-iii.py
diff --git a/problems/best-time-to-buy-and-sell-stock-with-cooldown.py b/problems/python/best-time-to-buy-and-sell-stock-with-cooldown.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/best-time-to-buy-and-sell-stock-with-cooldown.py
rename to problems/python/best-time-to-buy-and-sell-stock-with-cooldown.py
diff --git a/problems/best-time-to-buy-and-sell-stock.py b/problems/python/best-time-to-buy-and-sell-stock.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/best-time-to-buy-and-sell-stock.py
rename to problems/python/best-time-to-buy-and-sell-stock.py
diff --git a/problems/big-countries.sql b/problems/python/big-countries.sql
old mode 100644
new mode 100755
similarity index 100%
rename from problems/big-countries.sql
rename to problems/python/big-countries.sql
diff --git a/problems/binary-search-tree-iterator.py b/problems/python/binary-search-tree-iterator.py
old mode 100644
new mode 100755
similarity index 71%
rename from problems/binary-search-tree-iterator.py
rename to problems/python/binary-search-tree-iterator.py
index 65557ce..48d1458
--- a/problems/binary-search-tree-iterator.py
+++ b/problems/python/binary-search-tree-iterator.py
@@ -61,4 +61,28 @@ def inOrderTraverse(root):
         #do something
         print node.val
         
-        node = node.right
\ No newline at end of file
+        node = node.right
+
+
+
+class BSTIterator(object):
+
+    def __init__(self, root):
+        self.stack = []
+        self.node = root
+        
+        
+
+    def next(self):
+        while self.node:
+            self.stack.append(self.node)
+            self.node = self.node.left
+        self.node = self.stack.pop()
+        returnVal = self.node.val
+        self.node = self.node.right
+        return returnVal
+        
+        
+
+    def hasNext(self):
+        return self.node or self.stack
\ No newline at end of file
diff --git a/problems/binary-search.py b/problems/python/binary-search.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-search.py
rename to problems/python/binary-search.py
diff --git a/problems/python/binary-subarrays-with-sum.py b/problems/python/binary-subarrays-with-sum.py
new file mode 100755
index 0000000..dafb077
--- /dev/null
+++ b/problems/python/binary-subarrays-with-sum.py
@@ -0,0 +1,19 @@
+class Solution(object):
+    def numSubarraysWithSum(self, nums, goal):
+        #number of subarrays with sum at most "goal"
+        def atMost(goal):
+            ans = 0
+            total = 0
+            i = 0
+            
+            for j, num in enumerate(nums):
+                if num==1: total += 1
+                
+                while i<len(nums) and total>goal:
+                    if nums[i]==1: total -= 1
+                    i += 1
+                
+                ans += j-i+1 #number of subarrays that can generate from nums[i~j]
+            return ans
+        
+        return atMost(goal)-atMost(goal-1) if goal>0 else atMost(goal)
\ No newline at end of file
diff --git a/problems/binary-tree-inorder-traversal.py b/problems/python/binary-tree-inorder-traversal.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-inorder-traversal.py
rename to problems/python/binary-tree-inorder-traversal.py
diff --git a/problems/binary-tree-level-order-traversal-ii.py b/problems/python/binary-tree-level-order-traversal-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-level-order-traversal-ii.py
rename to problems/python/binary-tree-level-order-traversal-ii.py
diff --git a/problems/binary-tree-level-order-traversal.py b/problems/python/binary-tree-level-order-traversal.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-level-order-traversal.py
rename to problems/python/binary-tree-level-order-traversal.py
diff --git a/problems/binary-tree-longest-consecutive-sequence.py b/problems/python/binary-tree-longest-consecutive-sequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-longest-consecutive-sequence.py
rename to problems/python/binary-tree-longest-consecutive-sequence.py
diff --git a/problems/binary-tree-maximum-path-sum.py b/problems/python/binary-tree-maximum-path-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-maximum-path-sum.py
rename to problems/python/binary-tree-maximum-path-sum.py
diff --git a/problems/binary-tree-paths.py b/problems/python/binary-tree-paths.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-paths.py
rename to problems/python/binary-tree-paths.py
diff --git a/problems/binary-tree-pruning.py b/problems/python/binary-tree-pruning.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-pruning.py
rename to problems/python/binary-tree-pruning.py
diff --git a/problems/binary-tree-right-side-view.py b/problems/python/binary-tree-right-side-view.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-tree-right-side-view.py
rename to problems/python/binary-tree-right-side-view.py
diff --git a/problems/binary-tree-vertical-order-traversal.py b/problems/python/binary-tree-vertical-order-traversal.py
old mode 100644
new mode 100755
similarity index 51%
rename from problems/binary-tree-vertical-order-traversal.py
rename to problems/python/binary-tree-vertical-order-traversal.py
index ac0f3c6..1510ac0
--- a/problems/binary-tree-vertical-order-traversal.py
+++ b/problems/python/binary-tree-vertical-order-traversal.py
@@ -24,4 +24,27 @@ def verticalOrder(self, root):
             if node.left: q.append((node.left, x-1))
             if node.right: q.append((node.right, x+1))
             
-        return [ans[x] for x in xrange(minX, maxX+1)]
\ No newline at end of file
+        return [ans[x] for x in xrange(minX, maxX+1)]
+
+
+class Solution(object):
+    def verticalOrder(self, root):
+        if not root: return []
+        ans = []
+        minX = float('inf')
+        maxX = float('-inf')
+        locations = collections.defaultdict(list)
+        q = collections.deque([(root, 0)])
+        
+        while q:
+            node, x = q.popleft()
+            locations[x].append(node.val)
+            minX = min(minX, x)
+            maxX = max(maxX, x)
+            if node.left: q.append((node.left, x-1))
+            if node.right: q.append((node.right, x+1))
+        
+        for x in xrange(minX, maxX+1):
+            if locations[x]: ans.append(locations[x])
+                
+        return ans
\ No newline at end of file
diff --git a/problems/binary-watch.py b/problems/python/binary-watch.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/binary-watch.py
rename to problems/python/binary-watch.py
diff --git a/problems/python/buildings-with-an-ocean-view.py b/problems/python/buildings-with-an-ocean-view.py
new file mode 100755
index 0000000..0025b13
--- /dev/null
+++ b/problems/python/buildings-with-an-ocean-view.py
@@ -0,0 +1,16 @@
+"""
+Traverse from the right and keep track of the highest building.
+
+Time: O(N)
+Space: O(1)
+"""
+class Solution(object):
+    def findBuildings(self, heights):
+        ans = []
+        currMaxHeight = 0
+        for i in xrange(len(heights)-1, -1, -1):
+            h = heights[i]
+            if h>currMaxHeight: ans.append(i)
+            currMaxHeight = max(currMaxHeight, h)
+        
+        return reversed(ans)
\ No newline at end of file
diff --git a/problems/burst-balloons.py b/problems/python/burst-balloons.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/burst-balloons.py
rename to problems/python/burst-balloons.py
diff --git a/problems/python/campus-bikes-ii.py b/problems/python/campus-bikes-ii.py
new file mode 100755
index 0000000..a292d91
--- /dev/null
+++ b/problems/python/campus-bikes-ii.py
@@ -0,0 +1,35 @@
+"""
+state[i] := ith bike has been selected.
+For example, there are M bikes, 0000, 0100, 0110, 1111.....
+
+Time: O(ELogE)
+Space: O(E)
+"""
+class Solution(object):
+    def assignBikes(self, workers, bikes):
+        M = len(bikes)
+        N = len(workers)
+        costs = [[0]*N for _ in xrange(M)]
+        
+        for i in xrange(M):
+            for j in xrange(N):
+                costs[i][j] = abs(workers[j][0]-bikes[i][0])+abs(workers[j][1]-bikes[i][1])
+        
+        pq = [(0, '0'*M)]
+        visited = set()
+        while pq:
+            cost, state = heapq.heappop(pq)
+            if state in visited: continue
+            visited.add(state)
+            
+            j = state.count('1') #j users have selected bikes
+            if j>=N: return cost
+            
+            for i in xrange(M):
+                if state[i]=='1': continue
+                
+                nextState = state[:i] + '1' + state[i+1:]
+                if nextState in visited: continue
+                heapq.heappush(pq, (cost+costs[i][j], nextState))
+        
+        return float('inf')
\ No newline at end of file
diff --git a/problems/candy.py b/problems/python/candy.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/candy.py
rename to problems/python/candy.py
diff --git a/problems/capacity-to-ship-packages-within-d-days.py b/problems/python/capacity-to-ship-packages-within-d-days.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/capacity-to-ship-packages-within-d-days.py
rename to problems/python/capacity-to-ship-packages-within-d-days.py
diff --git a/problems/cheapest-flights-within-k-stops.py b/problems/python/cheapest-flights-within-k-stops.py
old mode 100644
new mode 100755
similarity index 97%
rename from problems/cheapest-flights-within-k-stops.py
rename to problems/python/cheapest-flights-within-k-stops.py
index 28b2927..eb785b3
--- a/problems/cheapest-flights-within-k-stops.py
+++ b/problems/python/cheapest-flights-within-k-stops.py
@@ -23,16 +23,19 @@ class Solution1(object):
 	def findCheapestPrice(self, n, flights, src, dst, K):
 		graph = collections.defaultdict(list)
 		pq = []
+		visited = set()
 
 		for u, v, w in flights: graph[u].append((w, v))
 
 		heapq.heappush(pq, (0, K+1, src))
 		while pq:
 			price, stops, city = heapq.heappop(pq)
+			visited.add(city)
 
 			if city is dst: return price
 			if stops>0:
 				for price_to_nei, nei in graph[city]:
+					if nei in visited: continue
 					heapq.heappush(pq, (price+price_to_nei, stops-1, nei))
 		return -1
 
diff --git a/problems/climbing-stairs.py b/problems/python/climbing-stairs.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/climbing-stairs.py
rename to problems/python/climbing-stairs.py
diff --git a/problems/clone-graph.py b/problems/python/clone-graph.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/clone-graph.py
rename to problems/python/clone-graph.py
diff --git a/problems/closest-binary-search-tree-value.py b/problems/python/closest-binary-search-tree-value.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/closest-binary-search-tree-value.py
rename to problems/python/closest-binary-search-tree-value.py
diff --git a/problems/coin-change.py b/problems/python/coin-change.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/coin-change.py
rename to problems/python/coin-change.py
diff --git a/problems/python/coloring-a-border.py b/problems/python/coloring-a-border.py
new file mode 100755
index 0000000..0b1ecc9
--- /dev/null
+++ b/problems/python/coloring-a-border.py
@@ -0,0 +1,43 @@
+"""
+A node is on the "border" if 
+1. It is on the actual border of the grid.
+Or
+2. Any of its neighbor has different color.
+
+BFS the grid starting from (row, col), if the node "isBorder" add it to nodesToColor.
+Color the nodes in the nodesToColor.
+
+Time: O(N)
+Space: O(N)
+"""
+class Solution(object):
+    def colorBorder(self, grid, row, col, color):
+        def isValid(i, j):
+            return i>=0 and j>=0 and i<len(grid) and j<len(grid[0])
+        
+        def isBorder(i, j, color):
+            if i==0 or i==len(grid)-1 or j==0 or j==len(grid[0])-1: return True
+            if any(grid[iNei][jNei]!=color for iNei, jNei in [(i+1, j), (i-1, j), (i, j+1), (i, j-1)]): return True
+            return False
+            
+        q = collections.deque([(row, col)])
+        nodesToColor = []
+        visited = set()
+        
+        while q:
+            i, j = q.popleft()
+            
+            if not isValid(i, j): continue
+            if grid[i][j]!=grid[row][col]: continue
+            if (i, j) in visited: continue
+            visited.add((i, j))
+            
+            if isBorder(i, j, grid[row][col]): nodesToColor.append((i, j))
+            
+            for iNext, jNext in [(i+1, j), (i-1, j), (i, j+1), (i, j-1)]:
+                q.append((iNext, jNext))
+        
+        for i, j in nodesToColor:
+            grid[i][j] = color
+        
+        return grid
\ No newline at end of file
diff --git a/problems/combination-sum-ii.py b/problems/python/combination-sum-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/combination-sum-ii.py
rename to problems/python/combination-sum-ii.py
diff --git a/problems/combination-sum-iii.py b/problems/python/combination-sum-iii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/combination-sum-iii.py
rename to problems/python/combination-sum-iii.py
diff --git a/problems/combination-sum-iv.py b/problems/python/combination-sum-iv.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/combination-sum-iv.py
rename to problems/python/combination-sum-iv.py
diff --git a/problems/combination-sum.py b/problems/python/combination-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/combination-sum.py
rename to problems/python/combination-sum.py
diff --git a/problems/combinations.py b/problems/python/combinations.py
old mode 100644
new mode 100755
similarity index 87%
rename from problems/combinations.py
rename to problems/python/combinations.py
index 49f7ebf..f3a0937
--- a/problems/combinations.py
+++ b/problems/python/combinations.py
@@ -83,6 +83,21 @@ def dfs(n_min, path):
 
 
 
+class Solution(object):
+    def combine(self, N, K):
+        def dfs(comb, start, N, K):
+            if len(comb)==K: ans.append(comb[:])
+            
+            for n in xrange(start, N+1):
+                comb.append(n)
+                dfs(comb, n+1, N, K)
+                comb.pop()
+        
+        ans = []
+        dfs([], 1, N, K)
+        return ans
+
+
 
 
 
diff --git a/problems/combine-two-tables.sql b/problems/python/combine-two-tables.sql
old mode 100644
new mode 100755
similarity index 100%
rename from problems/combine-two-tables.sql
rename to problems/python/combine-two-tables.sql
diff --git a/problems/compare-version-numbers.py b/problems/python/compare-version-numbers.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/compare-version-numbers.py
rename to problems/python/compare-version-numbers.py
diff --git a/problems/connecting-cities-with-minimum-cost.py b/problems/python/connecting-cities-with-minimum-cost.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/connecting-cities-with-minimum-cost.py
rename to problems/python/connecting-cities-with-minimum-cost.py
diff --git a/problems/consecutive-numbers-sum.py b/problems/python/consecutive-numbers-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/consecutive-numbers-sum.py
rename to problems/python/consecutive-numbers-sum.py
diff --git a/problems/construct-binary-tree-from-inorder-and-postorder-traversal.py b/problems/python/construct-binary-tree-from-inorder-and-postorder-traversal.py
old mode 100644
new mode 100755
similarity index 67%
rename from problems/construct-binary-tree-from-inorder-and-postorder-traversal.py
rename to problems/python/construct-binary-tree-from-inorder-and-postorder-traversal.py
index 1a46912..f8304fc
--- a/problems/construct-binary-tree-from-inorder-and-postorder-traversal.py
+++ b/problems/python/construct-binary-tree-from-inorder-and-postorder-traversal.py
@@ -68,4 +68,35 @@ def helper(i, j, k, l):
         
         index = {} #the index of inorder
         for i, n in enumerate(inorder): index[n] = i
-        return helper(0, len(inorder), 0, len(postorder))
\ No newline at end of file
+        return helper(0, len(inorder), 0, len(postorder))
+
+
+
+
+
+"""
+Inorder: [LEFT]root[RIGHT]
+Postorder: [LEFT][RIGHT]root
+As you can see, rootVal is the last val in postorder.
+
+i and j is the range of the inorder
+k and l is the range of the postorder
+Get the root
+Also recursively use the same method to get the left and right node, but this time we change the inorder and postorder.
+"""
+class Solution(object):
+    def buildTree(self, inorder, postorder):
+        def helper(i, j, k, l):
+            if i>j or k>l: return None
+            rootVal = postorder[l]
+            root = TreeNode(rootVal)
+            r = inorderIndex[rootVal]
+            leftLength = r-i
+            rightLength = j-r
+            root.left = helper(i, r-1, k, k+leftLength-1)
+            root.right = helper(r+1, j, k+leftLength, k+leftLength+rightLength-1)
+            return root
+            
+        inorderIndex = {}
+        for i, n in enumerate(inorder): inorderIndex[n] = i
+        return helper(0, len(inorder)-1, 0, len(postorder)-1)
diff --git a/problems/construct-binary-tree-from-preorder-and-inorder-traversal.py b/problems/python/construct-binary-tree-from-preorder-and-inorder-traversal.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/construct-binary-tree-from-preorder-and-inorder-traversal.py
rename to problems/python/construct-binary-tree-from-preorder-and-inorder-traversal.py
diff --git a/problems/container-with-most-water.py b/problems/python/container-with-most-water.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/container-with-most-water.py
rename to problems/python/container-with-most-water.py
diff --git a/problems/contains-duplicate-ii.py b/problems/python/contains-duplicate-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/contains-duplicate-ii.py
rename to problems/python/contains-duplicate-ii.py
diff --git a/problems/contains-duplicate-iii.py b/problems/python/contains-duplicate-iii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/contains-duplicate-iii.py
rename to problems/python/contains-duplicate-iii.py
diff --git a/problems/contains-duplicate.py b/problems/python/contains-duplicate.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/contains-duplicate.py
rename to problems/python/contains-duplicate.py
diff --git a/problems/python/continuous-subarray-sum.py b/problems/python/continuous-subarray-sum.py
new file mode 100755
index 0000000..75dd08b
--- /dev/null
+++ b/problems/python/continuous-subarray-sum.py
@@ -0,0 +1,35 @@
+class Solution(object):
+    def checkSubarraySum(self, nums, k):
+        prefixSumEndings = collections.defaultdict(list)
+        prefixSumEndings[0].append(-1)
+        
+        prefixSum = 0
+        for i, n in enumerate(nums):
+            prefixSum += n
+            
+            x = prefixSum/k
+            while x>=0:
+                p2 = prefixSum-k*x
+                if len(prefixSumEndings[p2])>0 and prefixSumEndings[p2][0]<i-1:
+                    return True
+                x -= 1
+            prefixSumEndings[prefixSum].append(i)
+        
+        return False
+
+class Solution(object):
+    def checkSubarraySum(self, nums, k):
+        if len(nums)==0 or len(nums)==1: return False
+        prefixSumKRemain = collections.defaultdict(list)
+        
+        prefixSum = 0
+        for i, n in enumerate(nums):
+            prefixSum += n
+            remain = prefixSum%k
+            
+            if prefixSum%k==0 and i>=1: return True
+            if len(prefixSumKRemain[remain])>0 and prefixSumKRemain[remain][0]<i-1:
+                return True
+            prefixSumKRemain[remain].append(i)
+            
+        return False
\ No newline at end of file
diff --git a/problems/convert-binary-search-tree-to-sorted-doubly-linked-list.py b/problems/python/convert-binary-search-tree-to-sorted-doubly-linked-list.py
old mode 100644
new mode 100755
similarity index 81%
rename from problems/convert-binary-search-tree-to-sorted-doubly-linked-list.py
rename to problems/python/convert-binary-search-tree-to-sorted-doubly-linked-list.py
index cfd441f..f813a37
--- a/problems/convert-binary-search-tree-to-sorted-doubly-linked-list.py
+++ b/problems/python/convert-binary-search-tree-to-sorted-doubly-linked-list.py
@@ -101,4 +101,30 @@ def inOrderTraverse(root):
         #do something
         print node.val
         
-        node = node.right
\ No newline at end of file
+        node = node.right
+
+"""
+Time: O(N)
+Space: O(LogN)
+"""
+class Solution(object):
+    def treeToDoublyList(self, root):
+        def helper(node):
+            ll = rr = node
+            if node.left:
+                ll, lr = helper(node.left)
+                node.left = lr
+                lr.right = node
+            
+            if node.right:
+                rl, rr = helper(node.right)
+                node.right = rl
+                rl.left = node
+            
+            return ll, rr
+        
+        if not root: return root
+        left, right = helper(root)
+        right.right = left
+        left.left = right
+        return left
\ No newline at end of file
diff --git a/problems/convert-sorted-array-to-binary-search-tree.py b/problems/python/convert-sorted-array-to-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/convert-sorted-array-to-binary-search-tree.py
rename to problems/python/convert-sorted-array-to-binary-search-tree.py
diff --git a/problems/convert-sorted-list-to-binary-search-tree.py b/problems/python/convert-sorted-list-to-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/convert-sorted-list-to-binary-search-tree.py
rename to problems/python/convert-sorted-list-to-binary-search-tree.py
diff --git a/problems/copy-list-with-random-pointer.py b/problems/python/copy-list-with-random-pointer.py
old mode 100644
new mode 100755
similarity index 67%
rename from problems/copy-list-with-random-pointer.py
rename to problems/python/copy-list-with-random-pointer.py
index 0790e26..ddf901e
--- a/problems/copy-list-with-random-pointer.py
+++ b/problems/python/copy-list-with-random-pointer.py
@@ -55,5 +55,38 @@ def copyRandomList(self, head):
 
 """
 Time: O(N). Two iteration. First iteration make a the clones. Second iteration setup the links.
-Space: O(1). No extra space except the cloned node.
-"""
\ No newline at end of file
+Space: O(N).
+"""
+
+
+"""
+Time: O(N)
+Space: O(1)
+"""
+class Solution(object):
+    def copyRandomList(self, head):
+        if not head: return head
+        curr = head
+        while curr:
+            nextCurr = curr.next
+            copy = Node(curr.val)
+            curr.next = copy
+            copy.next = nextCurr
+            curr = nextCurr
+        
+        copyHead = head.next
+
+        curr = head
+        while curr:
+            if curr.random: curr.next.random = curr.random.next
+            curr = curr.next.next
+        
+        curr = head
+        while curr:
+            nextCurr = curr.next.next
+            if nextCurr:
+                curr.next.next = nextCurr.next
+            curr.next = nextCurr
+            curr = nextCurr
+        
+        return copyHead
\ No newline at end of file
diff --git a/problems/python/count-all-valid-pickup-and-delivery-opti.py b/problems/python/count-all-valid-pickup-and-delivery-opti.py
new file mode 100755
index 0000000..8bfe82a
--- /dev/null
+++ b/problems/python/count-all-valid-pickup-and-delivery-opti.py
@@ -0,0 +1,5 @@
+class Solution(object):
+    def countOrders(self, n):
+        d = 1
+        for i in xrange(1, 2*n, 2): d*=i
+        return math.factorial(n)*d % (10**9 + 7)
\ No newline at end of file
diff --git a/problems/python/count-binary-substrings.py b/problems/python/count-binary-substrings.py
new file mode 100755
index 0000000..5a2520e
--- /dev/null
+++ b/problems/python/count-binary-substrings.py
@@ -0,0 +1,21 @@
+class Solution(object):
+    def countBinarySubstrings(self, s):
+        groups = []
+        count = 0
+        last = s[0]
+        ans = 0
+        
+        for bit in s:
+            if bit==last:
+                count += 1
+            else:
+                groups.append(count)
+                count = 1
+            last = bit
+        
+        groups.append(count)
+        
+        for i in xrange(1, len(groups)):
+            ans += min(groups[i], groups[i-1])
+        
+        return ans
\ No newline at end of file
diff --git a/problems/count-complete-tree-nodes.py b/problems/python/count-complete-tree-nodes.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/count-complete-tree-nodes.py
rename to problems/python/count-complete-tree-nodes.py
diff --git a/problems/python/count-number-of-nice-subarrays.py b/problems/python/count-number-of-nice-subarrays.py
new file mode 100755
index 0000000..33df804
--- /dev/null
+++ b/problems/python/count-number-of-nice-subarrays.py
@@ -0,0 +1,19 @@
+class Solution(object):
+    def numberOfSubarrays(self, nums, K):
+        def atMost(k):
+            oddCount = 0
+            ans = 0
+            i = 0
+            
+            for j in xrange(len(nums)):
+                if nums[j]%2!=0: oddCount += 1
+                
+                while oddCount>k:
+                    if nums[i]%2!=0: oddCount -= 1
+                    i += 1
+                
+                ans += j-i+1
+            
+            return ans
+        
+        return atMost(K)-atMost(K-1)
\ No newline at end of file
diff --git a/problems/count-unique-characters-of-all-substrings-of-a-given-string.py b/problems/python/count-unique-characters-of-all-substrings-of-a-given-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/count-unique-characters-of-all-substrings-of-a-given-string.py
rename to problems/python/count-unique-characters-of-all-substrings-of-a-given-string.py
diff --git a/problems/course-schedule-ii.py b/problems/python/course-schedule-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/course-schedule-ii.py
rename to problems/python/course-schedule-ii.py
diff --git a/problems/course-schedule.py b/problems/python/course-schedule.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/course-schedule.py
rename to problems/python/course-schedule.py
diff --git a/problems/python/custom-sort-string.py b/problems/python/custom-sort-string.py
new file mode 100755
index 0000000..2e63523
--- /dev/null
+++ b/problems/python/custom-sort-string.py
@@ -0,0 +1,20 @@
+"""
+Take a look at the char in s.
+For the char that is in the order, rearrange them to sortedChars with respect to the "order".
+For the char that is in not the order, put them in postString.
+"""
+class Solution(object):
+    def customSortString(self, order, s):
+        sortedChars = ''
+        counter = collections.Counter(s)
+        for c in order:
+            if c in order:
+                sortedChars += c*counter[c]
+        
+        orderSet = set(order)
+        postString = ''
+        for c in s:
+            if c not in orderSet:
+                postString += c
+        
+        return sortedChars+postString
\ No newline at end of file
diff --git a/problems/python/cutting-ribbons.py b/problems/python/cutting-ribbons.py
new file mode 100755
index 0000000..175e336
--- /dev/null
+++ b/problems/python/cutting-ribbons.py
@@ -0,0 +1,13 @@
+class Solution(object):
+    def maxLength(self, ribbons, k):
+        maxLen = max(ribbons)
+        minLen = 0
+        
+        while minLen<maxLen:
+            l = minLen + (maxLen-minLen+1)/2
+            
+            if sum([ribbonLen/l for ribbonLen in ribbons])>=k:
+                minLen = l
+            else:
+                maxLen = l-1
+        return maxLen
\ No newline at end of file
diff --git a/problems/data-stream-as-disjoint-intervals.py b/problems/python/data-stream-as-disjoint-intervals.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/data-stream-as-disjoint-intervals.py
rename to problems/python/data-stream-as-disjoint-intervals.py
diff --git a/problems/decode-string.py b/problems/python/decode-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/decode-string.py
rename to problems/python/decode-string.py
diff --git a/problems/decode-ways.py b/problems/python/decode-ways.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/decode-ways.py
rename to problems/python/decode-ways.py
diff --git a/problems/delete-and-earn.py b/problems/python/delete-and-earn.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/delete-and-earn.py
rename to problems/python/delete-and-earn.py
diff --git a/problems/python/delete-duplicate-folders-in-system.py b/problems/python/delete-duplicate-folders-in-system.py
new file mode 100755
index 0000000..512cd1a
--- /dev/null
+++ b/problems/python/delete-duplicate-folders-in-system.py
@@ -0,0 +1,42 @@
+class Node(object):
+    def __init__(self, val):
+        self.val = val
+        self.key = None
+        self.children = {}
+
+class Solution(object):
+    def deleteDuplicateFolder(self, paths):
+        def setKey(node):
+            node.key = ''
+            for c in sorted(node.children.keys()): #need to be sorted. so when child structs are the same, we won't generate different key from different iteration order.
+                setKey(node.children[c])
+                node.key += node.children[c].val + '|' + node.children[c].key + '|' #generate a key for each node. only considering its children structure. (see the "identical" definition, it does not consider the val of the node itself.)
+                
+            keyCount[node.key] += 1
+        
+        def addPath(node, path):
+            if node.children and keyCount[node.key]>1: return #leaf node does not apply to this rule
+            ans.append(path+[node.val])
+            for c in node.children:
+                addPath(node.children[c], path+[node.val])
+            
+            
+        ans = []
+        root = Node('/')
+        keyCount = collections.Counter()
+        
+        #build the tree
+        for path in paths:
+            node = root
+            for c in path:
+                if c not in node.children: node.children[c] = Node(c)
+                node = node.children[c]
+        
+        #set all nodes key recursively
+        setKey(root)
+
+        #build ans
+        for c in root.children:
+            addPath(root.children[c], [])
+        
+        return ans
\ No newline at end of file
diff --git a/problems/delete-node-in-a-bst.py b/problems/python/delete-node-in-a-bst.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/delete-node-in-a-bst.py
rename to problems/python/delete-node-in-a-bst.py
diff --git a/problems/delete-operation-for-two-strings.py b/problems/python/delete-operation-for-two-strings.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/delete-operation-for-two-strings.py
rename to problems/python/delete-operation-for-two-strings.py
diff --git a/problems/design-add-and-search-words-data-structure.py b/problems/python/design-add-and-search-words-data-structure.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/design-add-and-search-words-data-structure.py
rename to problems/python/design-add-and-search-words-data-structure.py
diff --git a/problems/design-in-memory-file-system.py b/problems/python/design-in-memory-file-system.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/design-in-memory-file-system.py
rename to problems/python/design-in-memory-file-system.py
diff --git a/problems/design-linked-list.py b/problems/python/design-linked-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/design-linked-list.py
rename to problems/python/design-linked-list.py
diff --git a/problems/design-tic-tac-toe.py b/problems/python/design-tic-tac-toe.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/design-tic-tac-toe.py
rename to problems/python/design-tic-tac-toe.py
diff --git a/problems/python/diagonal-traverse.py b/problems/python/diagonal-traverse.py
new file mode 100755
index 0000000..0510201
--- /dev/null
+++ b/problems/python/diagonal-traverse.py
@@ -0,0 +1,25 @@
+class Solution(object):
+    def findDiagonalOrder(self, mat):
+        def helper(i, j, reverse):
+            output = []
+            while 0<=i<len(mat) and 0<=j<len(mat[0]):
+                output.append(mat[i][j])
+                i += 1
+                j -= 1
+            return output if not reverse else reversed(output)
+        
+        
+        M = len(mat)
+        N = len(mat[0])
+        ans = []
+        reverse = True
+        
+        for j in xrange(N):
+            ans += helper(0, j, reverse)
+            reverse = not reverse
+        
+        for i in xrange(1, M):
+            ans += helper(i, N-1, reverse)
+            reverse = not reverse
+        
+        return ans
\ No newline at end of file
diff --git a/problems/diameter-of-binary-tree.py b/problems/python/diameter-of-binary-tree.py
old mode 100644
new mode 100755
similarity index 81%
rename from problems/diameter-of-binary-tree.py
rename to problems/python/diameter-of-binary-tree.py
index f30ea33..8af4f9b
--- a/problems/diameter-of-binary-tree.py
+++ b/problems/python/diameter-of-binary-tree.py
@@ -83,3 +83,23 @@ def getMaxDepth(node):
             
 #         return ans
 
+
+"""
+For each route in the binary tree, we can view it from the highest node in the route.
+So the length will be helper(node.left) + 1 + helper(node.right).
+"""
+class Solution(object):
+    def __init__(self):
+        self.ans = 0 #number of nodes in the longest route
+        
+    def diameterOfBinaryTree(self, root):
+	    #return the longest length to the leaf including itself
+        #also update self.ans
+        def helper(node):
+            r = helper(node.right) if node.right else 0
+            l = helper(node.left) if node.left else 0
+            self.ans = max(self.ans, l+1+r)
+            return max(l+1, r+1)
+        
+        helper(root)
+        return self.ans-1
\ No newline at end of file
diff --git a/problems/different-ways-to-add-parentheses.py b/problems/python/different-ways-to-add-parentheses.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/different-ways-to-add-parentheses.py
rename to problems/python/different-ways-to-add-parentheses.py
diff --git a/problems/distinct-subsequences.py b/problems/python/distinct-subsequences.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/distinct-subsequences.py
rename to problems/python/distinct-subsequences.py
diff --git a/problems/distribute-coins-in-binary-tree.py b/problems/python/distribute-coins-in-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/distribute-coins-in-binary-tree.py
rename to problems/python/distribute-coins-in-binary-tree.py
diff --git a/problems/domino-and-tromino-tiling.py b/problems/python/domino-and-tromino-tiling.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/domino-and-tromino-tiling.py
rename to problems/python/domino-and-tromino-tiling.py
diff --git a/problems/python/dot-product-of-two-sparse-vectors.py b/problems/python/dot-product-of-two-sparse-vectors.py
new file mode 100755
index 0000000..be69714
--- /dev/null
+++ b/problems/python/dot-product-of-two-sparse-vectors.py
@@ -0,0 +1,16 @@
+class SparseVector:
+    def __init__(self, nums):
+        self.indices = set()
+        self.values = {}
+        
+        for i, n in enumerate(nums):
+            if n!=0:
+                self.indices.add(i)
+                self.values[i] = n
+                
+    def dotProduct(self, vec):
+        products = 0
+        indices = self.indices.intersection(vec.indices)
+        for i in indices:
+            products += self.values[i]*vec.values[i]
+        return products
\ No newline at end of file
diff --git a/problems/dungeon-game.py b/problems/python/dungeon-game.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/dungeon-game.py
rename to problems/python/dungeon-game.py
diff --git a/problems/edit-distance.py b/problems/python/edit-distance.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/edit-distance.py
rename to problems/python/edit-distance.py
diff --git a/problems/egions-cut-by-slashes.py b/problems/python/egions-cut-by-slashes.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/egions-cut-by-slashes.py
rename to problems/python/egions-cut-by-slashes.py
diff --git a/problems/evaluate-division.py b/problems/python/evaluate-division.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/evaluate-division.py
rename to problems/python/evaluate-division.py
diff --git a/problems/evaluate-reverse-polish-notation.py b/problems/python/evaluate-reverse-polish-notation.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/evaluate-reverse-polish-notation.py
rename to problems/python/evaluate-reverse-polish-notation.py
diff --git a/problems/python/exclusive-time-of-functions.py b/problems/python/exclusive-time-of-functions.py
new file mode 100755
index 0000000..0f4fc4d
--- /dev/null
+++ b/problems/python/exclusive-time-of-functions.py
@@ -0,0 +1,24 @@
+class Solution(object):
+    def exclusiveTime(self, n, logs):
+        ans = [0]*n
+        data = []
+        for log in logs:
+            fid, action, t = log.split(':')
+            data.append([int(t), -1 if action=='start' else 1, int(fid)])
+        
+        data.sort()
+        
+        stack = []
+        for t, action, fid in data:
+            if action==-1:
+                if stack:
+                    prevT, _, prevFid = stack[-1]
+                    ans[prevFid] += t-prevT
+            elif action==1:
+                prevT, _, prevFid = stack.pop()
+                ans[fid] += t+1-prevT
+                if stack: stack[-1][0] = t+1
+                
+            if action==-1: stack.append([t, action, fid])
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python/expression-add-operators.py b/problems/python/expression-add-operators.py
new file mode 100755
index 0000000..64421a7
--- /dev/null
+++ b/problems/python/expression-add-operators.py
@@ -0,0 +1,23 @@
+class Solution(object):
+    def addOperators(self, num, target):
+        def dfs(num, curr, i):
+            if i==len(num):
+                if eval(curr[1:])==target: ans.append(curr[1:])
+                return
+            
+            if curr and curr[-1] in operators:
+                dfs(num, curr+num[i], i+1)
+            elif curr and curr[-1]=='0':
+                dfs(num, curr+'+', i)
+                dfs(num, curr+'-', i)
+                dfs(num, curr+'*', i)
+            else:
+                dfs(num, curr+'+', i)
+                dfs(num, curr+'-', i)
+                dfs(num, curr+'*', i)
+                dfs(num, curr+num[i], i+1)
+        
+        operators = set(['+', '-', '*'])
+        ans = []
+        dfs(num, '+', 0)
+        return ans
\ No newline at end of file
diff --git a/problems/filling-bookcase-shelves.py b/problems/python/filling-bookcase-shelves.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/filling-bookcase-shelves.py
rename to problems/python/filling-bookcase-shelves.py
diff --git a/problems/python/find-all-possible-recipes-from-given-supplies.py b/problems/python/find-all-possible-recipes-from-given-supplies.py
new file mode 100755
index 0000000..13772ac
--- /dev/null
+++ b/problems/python/find-all-possible-recipes-from-given-supplies.py
@@ -0,0 +1,33 @@
+"""
+Use Topological Sort
+1. Build an adj list and inbound.
+2. Starts from supplies topologically traverse the map.
+3. For each node popping up, if it is in recipes, add it to ans.
+
+Time: O(N), N is the number of "nodes" (ingredients+recipes)
+Space: O(N).
+"""
+class Solution(object):
+    def findAllRecipes(self, recipes, ingredients, supplies):
+        N = len(recipes)
+        adj = collections.defaultdict(list)
+        inbounds = collections.Counter()
+        q = collections.deque(supplies)
+        recipeSet = set(recipes)
+        ans = []
+        
+        for i, recipe in enumerate(recipes):
+            for ingredient in ingredients[i]:
+                adj[ingredient].append(recipe)
+                inbounds[recipe] += 1
+        
+        while q:
+            node = q.popleft()
+            
+            if node in recipeSet: ans.append(node)
+            
+            for nei in adj[node]:
+                inbounds[nei] -= 1
+                if inbounds[nei]==0: q.append(nei)
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python/find-and-replace-in-string.py b/problems/python/find-and-replace-in-string.py
new file mode 100755
index 0000000..4d74203
--- /dev/null
+++ b/problems/python/find-and-replace-in-string.py
@@ -0,0 +1,51 @@
+"""
+`p` is the index on s which already processed.
+"""
+class Solution(object):
+    def findReplaceString(self, s, indices, sources, targets):
+        p = -1
+        ans = ''
+        
+        memo = {}
+        for i, index in enumerate(indices):
+            memo[index] = (sources[i], targets[i])
+        
+        for i in xrange(len(s)):
+            if i<=p: continue
+            
+            if i in memo and (s[i:i+len(memo[i][0])] if i+len(memo[i][0])<len(s) else s[i:])==memo[i][0]:
+                ans += memo[i][1]
+                p = i+len(memo[i][0])-1
+            else:
+                ans += s[i]
+                p = i
+            
+        return ans
+
+"""
+Time: O(N). N is the length of `s`. The replacement won't be more than the length of the string, Let's assume it is N.
+Space: O(N).
+"""
+class Solution(object):
+    def findReplaceString(self, s, indices, sources, targets):
+        ans = ''
+        
+        replacement = {}
+        for i in xrange(len(indices)):
+            index = indices[i]
+            source = sources[i]
+            target = targets[i]
+            
+            if s[index:index+len(source)]!=source: continue
+            replacement[index] = (source, target)
+        
+        i = 0
+        while i<len(s):
+            if i not in replacement:
+                ans += s[i]
+                i += 1
+            else:
+                ans += replacement[i][1]
+                i += len(replacement[i][0])
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python/find-distance-in-a-binary-tree.py b/problems/python/find-distance-in-a-binary-tree.py
new file mode 100755
index 0000000..b87dd1b
--- /dev/null
+++ b/problems/python/find-distance-in-a-binary-tree.py
@@ -0,0 +1,28 @@
+class Solution(object):
+    def __init__(self):
+        self.lca = None
+        self.pHeight = 0
+        self.qHeight = 0
+        
+    def findDistance(self, root, p, q):
+        #find the count of p or q in node's subtree, set the first node we found that has count>=2 as lca.
+        def findCount(node):
+            if not node: return 0
+            count = 0
+            if node.val==p or node.val==q: count += 1
+            count += findPQCount(node.left)
+            count += findPQCount(node.right)
+            if count>=2 and not self.lca: self.lca = node
+            return count
+        
+        def findHeight(node, h):
+            if not node: return
+            if node.val==p: self.pHeight = h
+            if node.val==q: self.qHeight = h
+            if self.pHeight and self.qHeight: return
+            findHeight(node.left, h+1)
+            findHeight(node.right, h+1)
+            
+        findCount(root)
+        findHeight(self.lca, 0)
+        return self.qHeight + self.pHeight
\ No newline at end of file
diff --git a/problems/python/find-duplicate-subtrees.py b/problems/python/find-duplicate-subtrees.py
new file mode 100755
index 0000000..5729e8e
--- /dev/null
+++ b/problems/python/find-duplicate-subtrees.py
@@ -0,0 +1,16 @@
+class Solution(object):
+    def findDuplicateSubtrees(self, root):
+        def dfs(node):
+            if not node: return '#'
+            string = str(node.val) + ',' + dfs(node.left) + ',' + dfs(node.right)
+            data[string].append(node)
+            return string
+        
+        data = collections.defaultdict(list)
+        ans = []
+        
+        dfs(root)
+        
+        for s in data:
+            if len(data[s])>=2: ans.append(data[s][0])
+        return ans
\ No newline at end of file
diff --git a/problems/find-eventual-safe-states.py b/problems/python/find-eventual-safe-states.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-eventual-safe-states.py
rename to problems/python/find-eventual-safe-states.py
diff --git a/problems/find-first-and-last-position-of-element-in-sorted-array.py b/problems/python/find-first-and-last-position-of-element-in-sorted-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-first-and-last-position-of-element-in-sorted-array.py
rename to problems/python/find-first-and-last-position-of-element-in-sorted-array.py
diff --git a/problems/python/find-k-closest-elements.py b/problems/python/find-k-closest-elements.py
new file mode 100755
index 0000000..802279f
--- /dev/null
+++ b/problems/python/find-k-closest-elements.py
@@ -0,0 +1,20 @@
+class Solution(object):
+    def findClosestElements(self, arr, K, X):
+        def isCloserThan(n1, n2, x):
+            return abs(x-n1)<abs(x-n2) or (abs(x-n1)==abs(x-n2) and n1<n2)
+                
+        output1 = []
+        output2 = []
+        
+        r = bisect.bisect_left(arr, X)
+        l = r-1
+        
+        while len(output1)+len(output2)<K:
+            if r>=len(arr) or (l>=0 and isCloserThan(arr[l], arr[r], X)):
+                output1.append(arr[l])
+                l -= 1
+            else:
+                output2.append(arr[r])
+                r += 1
+        
+        return output1[::-1]+output2
diff --git a/problems/find-k-pairs-with-smallest-sums.py b/problems/python/find-k-pairs-with-smallest-sums.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-k-pairs-with-smallest-sums.py
rename to problems/python/find-k-pairs-with-smallest-sums.py
diff --git a/problems/find-leaves-of-binary-tree.py b/problems/python/find-leaves-of-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-leaves-of-binary-tree.py
rename to problems/python/find-leaves-of-binary-tree.py
diff --git a/problems/find-median-from-data-stream.py b/problems/python/find-median-from-data-stream.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-median-from-data-stream.py
rename to problems/python/find-median-from-data-stream.py
diff --git a/problems/find-minimum-in-rotated-sorted-array-ii.py b/problems/python/find-minimum-in-rotated-sorted-array-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-minimum-in-rotated-sorted-array-ii.py
rename to problems/python/find-minimum-in-rotated-sorted-array-ii.py
diff --git a/problems/find-minimum-in-rotated-sorted-array.py b/problems/python/find-minimum-in-rotated-sorted-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-minimum-in-rotated-sorted-array.py
rename to problems/python/find-minimum-in-rotated-sorted-array.py
diff --git a/problems/find-mode-in-binary-search-tree.py b/problems/python/find-mode-in-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-mode-in-binary-search-tree.py
rename to problems/python/find-mode-in-binary-search-tree.py
diff --git a/problems/python/find-original-array-from-doubled-array,py b/problems/python/find-original-array-from-doubled-array,py
new file mode 100755
index 0000000..e9e2726
--- /dev/null
+++ b/problems/python/find-original-array-from-doubled-array,py
@@ -0,0 +1,26 @@
+"""
+Time: O(NLogN)
+Space: O(N)
+"""
+class Solution(object):
+    def findOriginalArray(self, changed):
+        if len(changed)%2!=0: return []
+        
+        ans = []
+        counter = collections.Counter() #store the count of the doubled number
+        count = 0 #sum of count in counter
+        
+        changed.sort() #need to be sorted, otherwise we cannot identify which number is orginal or it is doubled.
+        
+        for num in changed:
+            if counter[num]>0:
+                #num is a doubled num
+                counter[num] -= 1
+                count -= 1
+                ans.append(num/2)
+            else:
+                #num is an original num
+                counter[num*2] += 1
+                count += 1
+        
+        return ans if count==0 else []
\ No newline at end of file
diff --git a/problems/find-peak-element.py b/problems/python/find-peak-element.py
old mode 100644
new mode 100755
similarity index 78%
rename from problems/find-peak-element.py
rename to problems/python/find-peak-element.py
index 5b9a8b3..aeb4726
--- a/problems/find-peak-element.py
+++ b/problems/python/find-peak-element.py
@@ -54,5 +54,24 @@ def findPeakElement(self, nums):
         return l
                 
                 
-            
+
+"""
+[l, r] is the possible range.
+Keep decreasing the range unsing binary search until l==r.
+Pay attention to
+m = l+(r-l+1)/2
+Sometimes you need m = l+(r-l)/2 to avoid infinite loop.
+"""
+class Solution(object):
+    def findPeakElement(self, nums):
+        l = 0
+        r = len(nums)-1
+        
+        while l<r:
+            m = l+(r-l+1)/2
+            if nums[m-1]<nums[m]:
+                l = m
+            else:
+                r = m-1
+        return l
         
\ No newline at end of file
diff --git a/problems/find-the-duplicate-number.py b/problems/python/find-the-duplicate-number.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/find-the-duplicate-number.py
rename to problems/python/find-the-duplicate-number.py
diff --git a/problems/first-bad-version.py b/problems/python/first-bad-version.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/first-bad-version.py
rename to problems/python/first-bad-version.py
diff --git a/problems/first-missing-positive.py b/problems/python/first-missing-positive.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/first-missing-positive.py
rename to problems/python/first-missing-positive.py
diff --git a/problems/first-unique-character-in-a-string.py b/problems/python/first-unique-character-in-a-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/first-unique-character-in-a-string.py
rename to problems/python/first-unique-character-in-a-string.py
diff --git a/problems/fizz-buzz.py b/problems/python/fizz-buzz.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/fizz-buzz.py
rename to problems/python/fizz-buzz.py
diff --git a/problems/flatten-binary-tree-to-linked-list.py b/problems/python/flatten-binary-tree-to-linked-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/flatten-binary-tree-to-linked-list.py
rename to problems/python/flatten-binary-tree-to-linked-list.py
diff --git a/problems/python/flip-binary-tree-to-match-preorder-traversal.py b/problems/python/flip-binary-tree-to-match-preorder-traversal.py
new file mode 100755
index 0000000..8727e28
--- /dev/null
+++ b/problems/python/flip-binary-tree-to-match-preorder-traversal.py
@@ -0,0 +1,23 @@
+class Solution(object):
+    def flipMatchVoyage(self, root, voyage):
+        def helper(node):
+            if not node: return
+            if node.val!=voyage[self.i]:
+                self.valid = False
+                return
+            
+            self.i += 1
+            if node.left and node.left.val!=voyage[self.i]:
+                self.ans.append(node.val)
+                helper(node.right)
+                helper(node.left)
+                
+            else:
+                helper(node.left)
+                helper(node.right)
+        
+        self.ans = []
+        self.i = 0
+        self.valid = True
+        helper(root)
+        return self.ans if self.valid else [-1]
\ No newline at end of file
diff --git a/problems/flip-string-to-monotone-increasing.py b/problems/python/flip-string-to-monotone-increasing.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/flip-string-to-monotone-increasing.py
rename to problems/python/flip-string-to-monotone-increasing.py
diff --git a/problems/flood-fill.py b/problems/python/flood-fill.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/flood-fill.py
rename to problems/python/flood-fill.py
diff --git a/problems/friend-circles.py b/problems/python/friend-circles.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/friend-circles.py
rename to problems/python/friend-circles.py
diff --git a/problems/fruit-into-baskets.py b/problems/python/fruit-into-baskets.py
old mode 100644
new mode 100755
similarity index 75%
rename from problems/fruit-into-baskets.py
rename to problems/python/fruit-into-baskets.py
index 29bb77c..8645ae0
--- a/problems/fruit-into-baskets.py
+++ b/problems/python/fruit-into-baskets.py
@@ -44,3 +44,26 @@ def totalFruit(self, tree):
             counter = max(counter, i-start+1)
             mark[t] = i
         return counter
+
+
+"""
+Find the length of longest subarray that at most has 2 unique number.
+"""
+class Solution(object):
+    def totalFruit(self, fruits):
+        ans = 0
+        uniqueFruits = 0
+        i = 0
+        counter = collections.Counter()
+        
+        for j, fruit in enumerate(fruits):
+            counter[fruit] += 1
+            if counter[fruit]==1: uniqueFruits += 1
+            
+            while uniqueFruits>2:
+                counter[fruits[i]] -= 1
+                if counter[fruits[i]]==0: uniqueFruits -= 1
+                i += 1
+            
+            ans = max(ans, j-i+1)
+        return ans
\ No newline at end of file
diff --git a/problems/game-of-life.py b/problems/python/game-of-life.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/game-of-life.py
rename to problems/python/game-of-life.py
diff --git a/problems/generate-parentheses.py b/problems/python/generate-parentheses.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/generate-parentheses.py
rename to problems/python/generate-parentheses.py
diff --git a/problems/graph-valid-tree.py b/problems/python/graph-valid-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/graph-valid-tree.py
rename to problems/python/graph-valid-tree.py
diff --git a/problems/greatest-sum-divisible-by-three.py b/problems/python/greatest-sum-divisible-by-three.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/greatest-sum-divisible-by-three.py
rename to problems/python/greatest-sum-divisible-by-three.py
diff --git a/problems/group-anagrams.py b/problems/python/group-anagrams.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/group-anagrams.py
rename to problems/python/group-anagrams.py
diff --git a/problems/python/group-shifted-strings.py b/problems/python/group-shifted-strings.py
new file mode 100755
index 0000000..d5ac664
--- /dev/null
+++ b/problems/python/group-shifted-strings.py
@@ -0,0 +1,20 @@
+class Solution(object):
+    def groupStrings(self, strings):
+        def getHash(string):
+            h = ''
+            offset = getNumByChar(string[0])*-1
+            for c in string:
+                h += getCharByNum((getNumByChar(c)+offset) if (getNumByChar(c)+offset)>=0 else 26+(getNumByChar(c)+offset))
+            return h
+        
+        def getNumByChar(letter):
+            return ord(letter) - 97
+
+        def getCharByNum(pos):
+            return chr(pos + 97)
+        
+        groups = collections.defaultdict(list)
+        for string in strings:
+            h = getHash(string)
+            groups[h].append(string)
+        return groups.values()
\ No newline at end of file
diff --git a/problems/guess-number-higher-or-lower-ii.py b/problems/python/guess-number-higher-or-lower-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/guess-number-higher-or-lower-ii.py
rename to problems/python/guess-number-higher-or-lower-ii.py
diff --git a/problems/guess-number-higher-or-lower.py b/problems/python/guess-number-higher-or-lower.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/guess-number-higher-or-lower.py
rename to problems/python/guess-number-higher-or-lower.py
diff --git a/problems/python/guess-the-word.py b/problems/python/guess-the-word.py
new file mode 100755
index 0000000..278a659
--- /dev/null
+++ b/problems/python/guess-the-word.py
@@ -0,0 +1,32 @@
+# """
+# This is Master's API interface.
+# You should not implement it, or speculate about its implementation
+# """
+#class Master(object):
+#    def guess(self, word):
+#        """
+#        :type word: str
+#        :rtype int
+#        """
+
+class Solution(object):
+    def findSecretWord(self, wordlist, master):
+        def similarity(w1, w2):
+            count = 0
+            for i in xrange(6):
+                if w1[i]==w2[i]: count += 1
+            return count
+        
+        for _ in xrange(10):
+            
+            temp = []
+            
+            word = random.choice(wordlist)
+            k = master.guess(word)
+            if k==6: return
+            
+            for otherWord in wordlist:
+                if otherWord==word: continue
+                if similarity(word, otherWord)==k: temp.append(otherWord)
+            
+            wordlist = temp
\ No newline at end of file
diff --git a/problems/h-index-ii.py b/problems/python/h-index-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/h-index-ii.py
rename to problems/python/h-index-ii.py
diff --git a/problems/h-index.py b/problems/python/h-index.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/h-index.py
rename to problems/python/h-index.py
diff --git a/problems/hamming-distance.py b/problems/python/hamming-distance.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/hamming-distance.py
rename to problems/python/hamming-distance.py
diff --git a/problems/house-robber-ii.py b/problems/python/house-robber-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/house-robber-ii.py
rename to problems/python/house-robber-ii.py
diff --git a/problems/house-robber-iii.py b/problems/python/house-robber-iii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/house-robber-iii.py
rename to problems/python/house-robber-iii.py
diff --git a/problems/house-robber.py b/problems/python/house-robber.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/house-robber.py
rename to problems/python/house-robber.py
diff --git a/problems/implement-trie-prefix-tree.py b/problems/python/implement-trie-prefix-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/implement-trie-prefix-tree.py
rename to problems/python/implement-trie-prefix-tree.py
diff --git a/problems/increasing-triplet-subsequence.py b/problems/python/increasing-triplet-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/increasing-triplet-subsequence.py
rename to problems/python/increasing-triplet-subsequence.py
diff --git a/problems/inorder-successor-in-bst-ii.py b/problems/python/inorder-successor-in-bst-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/inorder-successor-in-bst-ii.py
rename to problems/python/inorder-successor-in-bst-ii.py
diff --git a/problems/inorder-successor-in-bst.py b/problems/python/inorder-successor-in-bst.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/inorder-successor-in-bst.py
rename to problems/python/inorder-successor-in-bst.py
diff --git a/problems/insert-interval.py b/problems/python/insert-interval.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/insert-interval.py
rename to problems/python/insert-interval.py
diff --git a/problems/insert-into-a-binary-search-tree.py b/problems/python/insert-into-a-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/insert-into-a-binary-search-tree.py
rename to problems/python/insert-into-a-binary-search-tree.py
diff --git a/problems/python/insert-into-a-sorted-circular-linked-list.py b/problems/python/insert-into-a-sorted-circular-linked-list.py
new file mode 100755
index 0000000..9a4a067
--- /dev/null
+++ b/problems/python/insert-into-a-sorted-circular-linked-list.py
@@ -0,0 +1,42 @@
+class Solution(object):
+    def insert(self, random, insertVal):
+        def allValSame(node):
+            curr = node.next
+            while curr!=node:
+                if curr.val!=node.val: return False
+                curr = curr.next
+            return True
+                
+        newNode = Node(insertVal)
+        
+        #handle null linked list
+        if not random:
+            newNode.next = newNode
+            return newNode
+        
+        #handle linked list with val all the same
+        if allValSame(random):
+            temp = random.next
+            random.next = newNode
+            newNode.next = temp
+            return random
+        
+        #find head and tail, head is the min val node, tail is the max val node.
+        curr = random
+        while curr.val<=curr.next.val:
+            curr = curr.next
+        tail = curr
+        head = curr.next
+        
+        #insert new node
+        if insertVal>=tail.val or insertVal<=head.val:
+            tail.next = newNode
+            newNode.next = head
+        else:
+            curr = head
+            while not curr.val<=insertVal<=curr.next.val: curr = curr.next
+            temp = curr.next
+            curr.next = newNode
+            newNode.next = temp
+        
+        return random
\ No newline at end of file
diff --git a/problems/insertion-sort-list.py b/problems/python/insertion-sort-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/insertion-sort-list.py
rename to problems/python/insertion-sort-list.py
diff --git a/problems/integer-to-english-words.py b/problems/python/integer-to-english-words.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/integer-to-english-words.py
rename to problems/python/integer-to-english-words.py
diff --git a/problems/interleaving-string.py b/problems/python/interleaving-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/interleaving-string.py
rename to problems/python/interleaving-string.py
diff --git a/problems/intersection-of-two-arrays-ii.py b/problems/python/intersection-of-two-arrays-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/intersection-of-two-arrays-ii.py
rename to problems/python/intersection-of-two-arrays-ii.py
diff --git a/problems/intersection-of-two-arrays.py b/problems/python/intersection-of-two-arrays.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/intersection-of-two-arrays.py
rename to problems/python/intersection-of-two-arrays.py
diff --git a/problems/invert-binary-tree.py b/problems/python/invert-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/invert-binary-tree.py
rename to problems/python/invert-binary-tree.py
diff --git a/problems/is-graph-bipartite.py b/problems/python/is-graph-bipartite.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/is-graph-bipartite.py
rename to problems/python/is-graph-bipartite.py
diff --git a/problems/is-subsequence.py b/problems/python/is-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/is-subsequence.py
rename to problems/python/is-subsequence.py
diff --git a/problems/isomorphic-strings.py b/problems/python/isomorphic-strings.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/isomorphic-strings.py
rename to problems/python/isomorphic-strings.py
diff --git a/problems/jewels-and-stones.py b/problems/python/jewels-and-stones.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/jewels-and-stones.py
rename to problems/python/jewels-and-stones.py
diff --git a/problems/jump-game.py b/problems/python/jump-game.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/jump-game.py
rename to problems/python/jump-game.py
diff --git a/problems/python/k-closest-points-to-origin.py b/problems/python/k-closest-points-to-origin.py
new file mode 100755
index 0000000..1f536c1
--- /dev/null
+++ b/problems/python/k-closest-points-to-origin.py
@@ -0,0 +1,131 @@
+class Solution(object):
+    def kClosest(self, points, k):
+        h = []
+        
+        for x, y in points:
+            d = (x**2+y**2)**0.5
+            if len(h)>=k and -h[0][0]>d:
+                heapq.heappop(h)
+                heapq.heappush(h, (-d, x, y))
+            elif len(h)>=k and -h[0][0]<=d:
+                pass
+            else:
+                heapq.heappush(h, (-d, x, y))
+        
+        return [(x, y) for _, x, y in h]
+
+
+"""
+1. Process `points` into `distances`.
+
+2. Binary search the "distance". For every distance:
+Split the elements in `distances` by distance
+Put the ones smaller to smaller.
+Put the ones larger to larger.
+If len(smaller)<=k, then all the elements in the smaller must belong to the `ans`, do the same thing to the larger.
+Else we ignore the larger and do the same thing to the smaller again.
+
+Time: O(N)
+The binary search range in average is N, N/2, N/4... = 2N
+Space: O(N)
+"""
+class Solution(object):
+    def kClosest(self, points, K):
+        #[1]
+        maxD = float('-inf')
+        minD = float('inf')
+        distances = []
+        for x, y in points:
+            distance = ((x**2+y**2)**0.5)
+            distances.append((distance, x, y))
+            maxD = max(maxD, distance)
+            minD = min(minD, distance)
+        
+        #[2]
+        ans = []
+        smaller = []
+        larger = []
+        while K>0:
+            #split distances into smaller and larger
+            distance = (maxD+minD)/2
+            for d, x, y in distances:
+                if d<=distance:
+                    smaller.append((d, x, y))
+                else:
+                    larger.append((d, x, y))
+            
+            if len(smaller)<=K:
+                ans += smaller
+                K -= len(smaller)
+                distances = larger
+                minD = distance
+                larger = []
+                smaller = []
+            else:
+                distances = smaller
+                maxD = distance
+                larger = []
+                smaller = []
+        
+        return [(x, y) for _, x, y in ans]
+
+
+
+"""
+Quick Select
+Time: O(N)
+Space: O(N), can be optimize to O(1).
+"""
+class Solution(object):
+    def kClosest(self, points, K):
+		"""
+		Start State:
+		i = s #next element after "SSS"s
+		t = s #unprocessed elements
+		j = e #next element after "LLLL"s
+
+		SSSPP?????LLL
+		i t   j
+
+		End State:
+		SSSPPPPPLLLLL
+		i   jt
+		"""
+        def quickSelect(distances, s, e, k):
+
+            pivot = distances[(s+e)/2][0]
+            i = s
+            j = e
+            t = s
+            
+            while t<=j:
+                if pivot<distances[t][0]:
+                    distances[t], distances[j] = distances[j], distances[t]
+                    j -= 1
+                elif pivot>distances[t][0]:
+                    distances[t], distances[i] = distances[i], distances[t]
+                    i += 1
+                    t += 1
+                else:
+                    t += 1
+            
+            if i-s>=k:
+                return quickSelect(distances, s, i, k)
+            elif j-s+1>=k:
+                return pivot
+            else:
+                return quickSelect(distances, t, e, k-(t-s))
+
+        distances = []
+        for x, y in points:
+            distance = ((x**2+y**2)**0.5)
+            distances.append((distance, x, y))
+        
+        kthSmallestDistance = quickSelect(distances, 0, len(distances)-1, K)
+        
+        ans = []
+        for d, x, y in distances:
+            if len(ans)==K: break
+            if d<=kthSmallestDistance: ans.append((x, y))
+        
+        return ans
\ No newline at end of file
diff --git a/problems/k-empty-slots.py b/problems/python/k-empty-slots.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/k-empty-slots.py
rename to problems/python/k-empty-slots.py
diff --git a/problems/keys-and-rooms.py b/problems/python/keys-and-rooms.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/keys-and-rooms.py
rename to problems/python/keys-and-rooms.py
diff --git a/problems/knight-dialer.py b/problems/python/knight-dialer.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/knight-dialer.py
rename to problems/python/knight-dialer.py
diff --git a/problems/knight-probability-in-chessboard.py b/problems/python/knight-probability-in-chessboard.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/knight-probability-in-chessboard.py
rename to problems/python/knight-probability-in-chessboard.py
diff --git a/problems/koko-eating-bananas.py b/problems/python/koko-eating-bananas.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/koko-eating-bananas.py
rename to problems/python/koko-eating-bananas.py
diff --git a/problems/kth-largest-element-in-an-array.py b/problems/python/kth-largest-element-in-an-array.py
old mode 100644
new mode 100755
similarity index 64%
rename from problems/kth-largest-element-in-an-array.py
rename to problems/python/kth-largest-element-in-an-array.py
index d12f59c..27c4b72
--- a/problems/kth-largest-element-in-an-array.py
+++ b/problems/python/kth-largest-element-in-an-array.py
@@ -63,4 +63,34 @@ def partition(A, l, r, p):
         
         k = len(nums)-k #redefine the problem to find the kth nums when sorted
         sortRange(nums, 0, len(nums)-1)
-        return nums[k]
\ No newline at end of file
+        return nums[k]
+
+
+#Quick Select
+class Solution(object):
+    def findKthLargest(self, nums, K):
+        def quickSelect(A, s, e, K):
+            pivot = A[(s+e)/2]
+            i = s
+            t = s
+            j = e
+            
+            while t<=j:
+                if A[t]<pivot:
+                    A[i], A[t] = A[t], A[i]
+                    i += 1
+                    t += 1
+                elif A[t]==pivot:
+                    t += 1
+                else:
+                    A[t], A[j] = A[j], A[t]
+                    j -= 1
+            
+            if e-j>=K:
+                return quickSelect(A, j+1, e, K)
+            elif e-i+1>=K:
+                return pivot
+            else:
+                return quickSelect(A, s, i-1, K-(e-(i-1)))
+            
+        return quickSelect(nums, 0, len(nums)-1, K)
\ No newline at end of file
diff --git a/problems/kth-smallest-element-in-a-bst.py b/problems/python/kth-smallest-element-in-a-bst.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/kth-smallest-element-in-a-bst.py
rename to problems/python/kth-smallest-element-in-a-bst.py
diff --git a/problems/kth-smallest-element-in-a-sorted-matrix.py b/problems/python/kth-smallest-element-in-a-sorted-matrix.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/kth-smallest-element-in-a-sorted-matrix.py
rename to problems/python/kth-smallest-element-in-a-sorted-matrix.py
diff --git a/problems/largest-1-bordered-square.py b/problems/python/largest-1-bordered-square.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/largest-1-bordered-square.py
rename to problems/python/largest-1-bordered-square.py
diff --git a/problems/largest-bst-subtree.py b/problems/python/largest-bst-subtree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/largest-bst-subtree.py
rename to problems/python/largest-bst-subtree.py
diff --git a/problems/largest-sum-of-averages.py b/problems/python/largest-sum-of-averages.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/largest-sum-of-averages.py
rename to problems/python/largest-sum-of-averages.py
diff --git a/problems/last-stone-weight-ii.py b/problems/python/last-stone-weight-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/last-stone-weight-ii.py
rename to problems/python/last-stone-weight-ii.py
diff --git a/problems/last-stone-weight.py b/problems/python/last-stone-weight.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/last-stone-weight.py
rename to problems/python/last-stone-weight.py
diff --git a/problems/least-number-of-unique-integers-after-k-removals.py b/problems/python/least-number-of-unique-integers-after-k-removals.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/least-number-of-unique-integers-after-k-removals.py
rename to problems/python/least-number-of-unique-integers-after-k-removals.py
diff --git a/problems/letter-case-permutation.py b/problems/python/letter-case-permutation.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/letter-case-permutation.py
rename to problems/python/letter-case-permutation.py
diff --git a/problems/letter-combinations-of-a-phone-number.py b/problems/python/letter-combinations-of-a-phone-number.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/letter-combinations-of-a-phone-number.py
rename to problems/python/letter-combinations-of-a-phone-number.py
diff --git a/problems/license-key-formatting.py b/problems/python/license-key-formatting.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/license-key-formatting.py
rename to problems/python/license-key-formatting.py
diff --git a/problems/linked-list-cycle-ii.py b/problems/python/linked-list-cycle-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/linked-list-cycle-ii.py
rename to problems/python/linked-list-cycle-ii.py
diff --git a/problems/linked-list-cycle.py b/problems/python/linked-list-cycle.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/linked-list-cycle.py
rename to problems/python/linked-list-cycle.py
diff --git a/problems/linked-list-random-node.py b/problems/python/linked-list-random-node.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/linked-list-random-node.py
rename to problems/python/linked-list-random-node.py
diff --git a/problems/python/logger-rate-limiter.py b/problems/python/logger-rate-limiter.py
new file mode 100755
index 0000000..0f5f7ad
--- /dev/null
+++ b/problems/python/logger-rate-limiter.py
@@ -0,0 +1,32 @@
+class Logger(object):
+    def __init__(self):
+        self.log = collections.Counter() #store the latest timestamp
+
+    def shouldPrintMessage(self, timestamp, message):
+        if message not in self.log or self.log[message]+10<=timestamp:
+            self.log[message] = timestamp
+            return True
+        else:
+            return False
+
+
+
+class Logger(object):
+
+    def __init__(self):
+        #stores the messages within 10 seconds
+        self.q = collections.deque()
+        self.set = set()
+        
+
+    def shouldPrintMessage(self, timestamp, message):
+        while self.q and timestamp-self.q[0][0]>=10:
+            time, msg = self.q.popleft()
+            self.set.remove(msg)
+        
+        if message not in self.set:
+            self.q.append((timestamp, message))
+            self.set.add(message)
+            return True
+        else:
+            return False
\ No newline at end of file
diff --git a/problems/longest-common-prefix.py b/problems/python/longest-common-prefix.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-common-prefix.py
rename to problems/python/longest-common-prefix.py
diff --git a/problems/longest-common-subsequence.py b/problems/python/longest-common-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-common-subsequence.py
rename to problems/python/longest-common-subsequence.py
diff --git a/problems/longest-consecutive-sequence.py b/problems/python/longest-consecutive-sequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-consecutive-sequence.py
rename to problems/python/longest-consecutive-sequence.py
diff --git a/problems/python/longest-increasing-path-in-a-matrix.py b/problems/python/longest-increasing-path-in-a-matrix.py
new file mode 100755
index 0000000..cd446d9
--- /dev/null
+++ b/problems/python/longest-increasing-path-in-a-matrix.py
@@ -0,0 +1,31 @@
+class Solution(object):
+    def longestIncreasingPath(self, matrix):
+        def getLongest(i, j):
+            if (i, j) in longest: return longest[(i, j)]
+            l = 1
+
+            #call getLongest to the neighbors that are larger than itself.
+            if i+1<M and matrix[i][j]<matrix[i+1][j]:
+                l = max(l, 1+getLongest(i+1, j))
+            if i-1>=0 and matrix[i][j]<matrix[i-1][j]:
+                l = max(l, 1+getLongest(i-1, j))
+            if j+1<N and matrix[i][j]<matrix[i][j+1]:
+                l = max(l, 1+getLongest(i, j+1))
+            if j-1>=0 and matrix[i][j]<matrix[i][j-1]:
+                l = max(l, 1+getLongest(i, j-1))
+            
+            longest[(i, j)] = l
+            self.ans = max(self.ans, l)
+            return l
+
+        M = len(matrix)
+        N = len(matrix[0])
+        
+        longest = {}
+        self.ans = float('-inf')
+        
+        for i in xrange(M):
+            for j in xrange(N):
+                getLongest(i, j)
+                
+        return self.ans
\ No newline at end of file
diff --git a/problems/longest-increasing-subsequence.py b/problems/python/longest-increasing-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-increasing-subsequence.py
rename to problems/python/longest-increasing-subsequence.py
diff --git a/problems/longest-palindromic-subsequence.py b/problems/python/longest-palindromic-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-palindromic-subsequence.py
rename to problems/python/longest-palindromic-subsequence.py
diff --git a/problems/longest-palindromic-substring.py b/problems/python/longest-palindromic-substring.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-palindromic-substring.py
rename to problems/python/longest-palindromic-substring.py
diff --git a/problems/longest-repeating-character-replacement.py b/problems/python/longest-repeating-character-replacement.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-repeating-character-replacement.py
rename to problems/python/longest-repeating-character-replacement.py
diff --git a/problems/longest-string-chain.py b/problems/python/longest-string-chain.py
old mode 100644
new mode 100755
similarity index 62%
rename from problems/longest-string-chain.py
rename to problems/python/longest-string-chain.py
index 79cb2f8..a21ef17
--- a/problems/longest-string-chain.py
+++ b/problems/python/longest-string-chain.py
@@ -66,3 +66,42 @@ def shortestPredecessor(word):
         return ans
 
 
+class Solution(object):
+    def longestStrChain(self, words):
+        def getLongestStringChain(word):
+            if word in history: return history[word]
+            if not orderedWords[len(word)+1]: return 1
+            
+            temp = 0
+            for word2 in orderedWords[len(word)+1]:
+                if isPredecessor(word2, word):
+                    temp = max(temp, getLongestStringChain(word2))
+            
+            history[word] = temp+1
+            return temp+1
+        
+        def isPredecessor(w2, w1):
+            if len(w2)!=len(w1)+1: return False
+            
+            j = 0
+            for c in w1:
+                found = False
+                while j<len(w2):
+                    if c==w2[j]:
+                        found = True
+                        j += 1
+                        break
+                    else:
+                        j += 1
+                if not found: return False
+
+            return True
+        
+        orderedWords = collections.defaultdict(list)
+        for word in words: orderedWords[len(word)].append(word)
+        history = {}
+        ans = 0
+        
+        for word in words:
+            ans = max(ans, getLongestStringChain(word))
+        return ans
\ No newline at end of file
diff --git a/problems/longest-substring-with-at-least-k-repeating-characters.py b/problems/python/longest-substring-with-at-least-k-repeating-characters.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-substring-with-at-least-k-repeating-characters.py
rename to problems/python/longest-substring-with-at-least-k-repeating-characters.py
diff --git a/problems/longest-substring-without-repeating-characters.py b/problems/python/longest-substring-without-repeating-characters.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-substring-without-repeating-characters.py
rename to problems/python/longest-substring-without-repeating-characters.py
diff --git a/problems/longest-univalue-path.py b/problems/python/longest-univalue-path.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/longest-univalue-path.py
rename to problems/python/longest-univalue-path.py
diff --git a/problems/lowest-common-ancestor-of-a-binary-search-tree.py b/problems/python/lowest-common-ancestor-of-a-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/lowest-common-ancestor-of-a-binary-search-tree.py
rename to problems/python/lowest-common-ancestor-of-a-binary-search-tree.py
diff --git a/problems/lowest-common-ancestor-of-a-binary-search.py b/problems/python/lowest-common-ancestor-of-a-binary-search.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/lowest-common-ancestor-of-a-binary-search.py
rename to problems/python/lowest-common-ancestor-of-a-binary-search.py
diff --git a/problems/python/lowest-common-ancestor-of-a-binary-tree-ii.py b/problems/python/lowest-common-ancestor-of-a-binary-tree-ii.py
new file mode 100755
index 0000000..8f07177
--- /dev/null
+++ b/problems/python/lowest-common-ancestor-of-a-binary-tree-ii.py
@@ -0,0 +1,17 @@
+class Solution(object):
+    def __init__(self):
+        self.ans = None
+        
+    def lowestCommonAncestor(self, root, p, q):
+        def dfs(node):
+            if not node: return 0
+            
+            count = 0
+            if node is p or node is q: count += 1
+            count += dfs(node.left)
+            count += dfs(node.right)
+            if count>=2 and not self.ans: self.ans = node
+            return count
+        
+        dfs(root)
+        return self.ans
diff --git a/problems/python/lowest-common-ancestor-of-a-binary-tree-iii.py b/problems/python/lowest-common-ancestor-of-a-binary-tree-iii.py
new file mode 100755
index 0000000..cfd14f7
--- /dev/null
+++ b/problems/python/lowest-common-ancestor-of-a-binary-tree-iii.py
@@ -0,0 +1,56 @@
+"""
+Time: O(H), H is the height of the tree.
+Space: O(1)
+"""
+class Solution(object):
+    def lowestCommonAncestor(self, p, q):
+        ancestorP = set()
+        ancestorQ = set()
+        
+        temp = p
+        while temp:
+            ancestorP.add(temp)
+            temp = temp.parent
+        
+        temp = q
+        while temp:
+            ancestorQ.add(temp)
+            temp = temp.parent
+        
+        commonAncestor = ancestorQ.intersection(ancestorP)
+        temp = q
+        while temp:
+            if temp in commonAncestor: return temp 
+            temp = temp.parent
+        return None
+
+
+"""
+Time: O(LogN)
+Space: O(LogN)
+
+Looking from backward, parents1 and parents2 will be the same at first, since they must have a common ancestor.
+Find the last the same parents.
+"""
+class Solution(object):
+    def lowestCommonAncestor(self, p, q):
+        parents1 = []
+        parents2 = []
+        
+        curr = p
+        while curr:
+            parents1.append(curr)
+            curr = curr.parent
+        
+        curr = q
+        while curr:
+            parents2.append(curr)
+            curr = curr.parent
+        
+        i = len(parents1)-1
+        j = len(parents2)-1
+        while i>=0 and j>=0 and parents1[i]==parents2[j]:
+            i -= 1
+            j -= 1
+        return parents1[i+1]
+    
diff --git a/problems/python/lowest-common-ancestor-of-a-binary-tree-iv.py b/problems/python/lowest-common-ancestor-of-a-binary-tree-iv.py
new file mode 100755
index 0000000..f7458a3
--- /dev/null
+++ b/problems/python/lowest-common-ancestor-of-a-binary-tree-iv.py
@@ -0,0 +1,18 @@
+class Solution(object):
+    def __init__(self):
+        self.ans = None
+        
+    def lowestCommonAncestor(self, root, nodes):
+        def dfs(node):
+            if not node: return 0
+            
+            count = 0
+            if node in nodes: count += 1
+            count += dfs(node.left)
+            count += dfs(node.right)
+            if count>=len(nodes) and not self.ans: self.ans = node    
+            return count
+        
+        nodes = set(nodes)
+        dfs(root)
+        return self.ans
\ No newline at end of file
diff --git a/problems/lowest-common-ancestor-of-a-binary-tree.py b/problems/python/lowest-common-ancestor-of-a-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/lowest-common-ancestor-of-a-binary-tree.py
rename to problems/python/lowest-common-ancestor-of-a-binary-tree.py
diff --git a/problems/python/lowest-common-ancestor-of-deepest-leaves.py b/problems/python/lowest-common-ancestor-of-deepest-leaves.py
new file mode 100755
index 0000000..cffade3
--- /dev/null
+++ b/problems/python/lowest-common-ancestor-of-deepest-leaves.py
@@ -0,0 +1,41 @@
+class Solution(object):
+    def __init__(self):
+        self.ans = None
+        
+    def lcaDeepestLeaves(self, root):
+        def checkCount(node, deepestNode):
+            if not node:
+                count = 0
+                if count==len(deepestNode) and not self.ans: self.ans = node
+                return 0
+            
+            if node in deepestNode:
+                count = 1
+                if count==len(deepestNode) and not self.ans: self.ans = node
+                return count
+            
+            leftCount = checkCount(node.left, deepestNode)
+            rightCount = checkCount(node.right, deepestNode)
+            
+            if leftCount+rightCount==len(deepestNode) and not self.ans: self.ans = node
+            return leftCount+rightCount
+            
+        q = collections.deque([(root, 0)])
+        q2 = collections.deque()
+        deepestNode = set([node for node, h in q])
+        
+        while q:
+            node, d = q.popleft()
+            if node.left: q2.append((node.left, d+1))
+            if node.right: q2.append((node.right, d+1))
+            if not q:
+                q = q2
+                if q: deepestNode = set([node for node, h in q])
+                q2 = collections.deque()
+        
+        checkCount(root, deepestNode)
+        return self.ans
+        
+                
+            
+        
\ No newline at end of file
diff --git a/problems/lru-cache.py b/problems/python/lru-cache.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/lru-cache.py
rename to problems/python/lru-cache.py
diff --git a/problems/majority-element-ii.py b/problems/python/majority-element-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/majority-element-ii.py
rename to problems/python/majority-element-ii.py
diff --git a/problems/majority-element.py b/problems/python/majority-element.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/majority-element.py
rename to problems/python/majority-element.py
diff --git a/problems/python/making-a-large-island.py b/problems/python/making-a-large-island.py
new file mode 100755
index 0000000..215aa2a
--- /dev/null
+++ b/problems/python/making-a-large-island.py
@@ -0,0 +1,51 @@
+"""
+For each "island" asign them a group id. Also calculate the group's size.
+Iterate all the zeros, update the ans.
+
+Time:O(MN)
+Space: O(MN) in the worst case.
+"""
+class Solution(object):
+    def largestIsland(self, grid):
+        def isValid(i, j, M, N):
+            return 0<=i<M and 0<=j<N
+        
+        def dfs(i, j, groupId):
+            if not isValid(i, j, M, N): return
+            if grid[i][j]!=1: return
+            grid[i][j] = groupId
+            groupIdToSize[groupId] += 1
+            for iNext, jNext in [(i+1, j), (i-1, j), (i, j-1), (i, j+1)]:
+                dfs(iNext, jNext, groupId)
+
+        M = len(grid)
+        N = len(grid[0])
+        
+        zeros = []
+        groupIdToSize = {}
+        groupId = 2
+        for i in xrange(M):
+            for j in xrange(N):
+                if grid[i][j]==1:
+                    groupIdToSize[groupId] = 0
+                    dfs(i, j, groupId)
+                    groupId += 1
+                elif grid[i][j]==0:
+                    zeros.append((i, j))
+                    
+        ans = max(groupIdToSize.values()) if groupIdToSize else 0
+        
+        for i, j in zeros:
+            neiGroupId = set()
+            neiSize = 0
+            for iNext, jNext in [(i+1, j), (i-1, j), (i, j-1), (i, j+1)]:
+                if not isValid(iNext, jNext, M, N): continue
+                if grid[iNext][jNext]>1:
+                    neiGroupId.add(grid[iNext][jNext])
+            
+            for groupId in list(neiGroupId):
+                neiSize += groupIdToSize[groupId]
+            
+            ans = max(ans, 1+neiSize)
+        
+        return ans
\ No newline at end of file
diff --git a/problems/max-area-of-island.py b/problems/python/max-area-of-island.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/max-area-of-island.py
rename to problems/python/max-area-of-island.py
diff --git a/problems/python/max-consecutive-ones-iii.py b/problems/python/max-consecutive-ones-iii.py
new file mode 100755
index 0000000..2ebd05c
--- /dev/null
+++ b/problems/python/max-consecutive-ones-iii.py
@@ -0,0 +1,15 @@
+class Solution(object):
+    def longestOnes(self, nums, k):
+        ans = 0
+        zeroCount = 0
+        i = 0
+        
+        for j, num in enumerate(nums):
+            if num==0: zeroCount += 1
+            
+            while zeroCount>k:
+                if nums[i]==0: zeroCount -= 1
+                i += 1
+            ans = max(ans, j-i+1)
+        
+        return ans
\ No newline at end of file
diff --git a/problems/max-stack.py b/problems/python/max-stack.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/max-stack.py
rename to problems/python/max-stack.py
diff --git a/problems/max-sum-of-rectangle-no-larger-than-k.py b/problems/python/max-sum-of-rectangle-no-larger-than-k.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/max-sum-of-rectangle-no-larger-than-k.py
rename to problems/python/max-sum-of-rectangle-no-larger-than-k.py
diff --git a/problems/python/maximal-square.py b/problems/python/maximal-square.py
new file mode 100755
index 0000000..9e6711b
--- /dev/null
+++ b/problems/python/maximal-square.py
@@ -0,0 +1,33 @@
+class Solution(object):
+    def maximalSquare(self, grid):
+        if not grid or not grid[0]: return 
+        M, N = len(grid), len(grid[0])
+        
+        dp = [[0 for _ in xrange(N+1)] for _ in xrange(M+1)]
+        ans = 0
+        for i in xrange(M):
+            for j in xrange(N):
+                if grid[i][j]=='1':
+                    dp[i+1][j+1] = min(dp[i][j], dp[i][j+1], dp[i+1][j])+1
+                    ans = max(ans, dp[i+1][j+1])
+        return ans**2
+
+
+
+class Solution(object):
+    def maximalSquare(self, matrix):
+        """
+        dp[i][j] := maximal square length with matrix[i-1][j-1] at the bottom right corner in the square
+        """
+        ans = 0
+        N = len(matrix)
+        M = len(matrix[0])
+        
+        dp = [[0 for _ in xrange(M+1)] for _ in xrange(N+1)]
+        
+        for i in xrange(1, N+1):
+            for j in xrange(1, M+1):
+                if matrix[i-1][j-1]=='1':
+                    dp[i][j] = min(dp[i][j-1], dp[i-1][j], dp[i-1][j-1])+1
+                    ans = max(ans, dp[i][j]**2)
+        return ans
\ No newline at end of file
diff --git a/problems/maximum-average-subtree.py b/problems/python/maximum-average-subtree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-average-subtree.py
rename to problems/python/maximum-average-subtree.py
diff --git a/problems/python/maximum-compatibility-score-sum.py b/problems/python/maximum-compatibility-score-sum.py
new file mode 100755
index 0000000..a0f744c
--- /dev/null
+++ b/problems/python/maximum-compatibility-score-sum.py
@@ -0,0 +1,44 @@
+"""
+Time: O(ELogE), E is the edge of the graph.
+Note that state[i] means if the ith student is matched or not. (for example M=4, 0000, 0010, 0111, 1111...)
+2^M is the number of states. So in this case E will be 2^M x M.
+
+Space: O(2^M)
+"""
+class Solution(object):
+    def maxCompatibilitySum(self, students, mentors):
+        M = len(students)
+        N = len(students[0])
+
+        #initialize reverseScores
+        reverseScores = [[0]*M for _ in xrange(M)]
+        for i in xrange(M):
+            for j in xrange(M):
+                reverseScore = 0
+                for k in xrange(N):
+                    if students[i][k]!=mentors[j][k]:
+                        reverseScore += 1
+                reverseScores[i][j] = reverseScore
+        
+        #Dijkstra
+        startState = '0'*M
+        endState = '1'*M
+        visited = set()
+        pq = [(0, startState)]
+        
+        while pq:
+            cost, state = heapq.heappop(pq)
+            if state in visited: continue
+            visited.add(state)
+            
+            if state==endState: return M*N-cost
+            
+            j = state.count('1')
+            for i in xrange(M):
+                if state[i]=='1': continue
+                
+                nextState = state[:i]+'1'+state[i+1:]
+                if nextState in visited: continue
+                heapq.heappush(pq, (cost+reverseScores[i][j], nextState))
+        
+        return -1
\ No newline at end of file
diff --git a/problems/maximum-depth-of-binary-tree.py b/problems/python/maximum-depth-of-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-depth-of-binary-tree.py
rename to problems/python/maximum-depth-of-binary-tree.py
diff --git a/problems/maximum-gap.py b/problems/python/maximum-gap.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-gap.py
rename to problems/python/maximum-gap.py
diff --git a/problems/maximum-length-of-repeated-subarray.py b/problems/python/maximum-length-of-repeated-subarray.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-length-of-repeated-subarray.py
rename to problems/python/maximum-length-of-repeated-subarray.py
diff --git a/problems/maximum-number-of-events-that-can-be-attended.py b/problems/python/maximum-number-of-events-that-can-be-attended.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-number-of-events-that-can-be-attended.py
rename to problems/python/maximum-number-of-events-that-can-be-attended.py
diff --git a/problems/python/maximum-number-of-points-with-cost.py b/problems/python/maximum-number-of-points-with-cost.py
new file mode 100755
index 0000000..c365817
--- /dev/null
+++ b/problems/python/maximum-number-of-points-with-cost.py
@@ -0,0 +1,63 @@
+"""
+dp[i][j] := considering from points[0]~points[i], the max points if choosing points[i][j]
+"""
+class Solution(object):
+    def maxPoints(self, points):
+        N = len(points)
+        M = len(points[0])
+        dp = [[0]*M for _ in xrange(N)]
+        
+        for i in xrange(N):
+            for j in xrange(M):
+                if i==0:
+                    dp[i][j] = points[i][j]
+                else:
+                    dp[i][j] = points[i][j]+max([dp[i-1][k] - abs(k-j) for k in xrange(M)])
+        return max(dp[N-1])
+
+    
+"""
+The above solution will take O(NM^2) in time.
+The bottle neck is for each j we need to traverse the whole last row.
+Let us see a little bit closer on `dp[i][j]`
+
+dp[i][j] = points[i][j]+max([dp[i-1][k] - abs(k-j) for k in xrange(M)])
+
+So, if j>=k (Part 1)
+points[i][j]+max([dp[i-1][k] - (j-k) for k in xrange(M)])
+points[i][j] - j + max([dp[i-1][k] + k) for k in xrange(M)])
+
+if k>=j (Part 2)
+points[i][j] + max([dp[i-1][k] - (k-j) for k in xrange(M)])
+points[i][j] + j + max([dp[i-1][k] - k) for k in xrange(M)])
+
+Since we cannot do a full scan
+why not we update the value from left to right for Part 1 and
+right to left for part 2
+With a variable call rollingMax to store the max.
+
+That way dp[i][j] will be updated as if we do a full scan.
+
+The time complexity will become O(NM)
+"""
+class Solution(object):
+    def maxPoints(self, points):
+        N = len(points)
+        M = len(points[0])
+        dp = [[0]*M for _ in xrange(N)]
+
+        for j in xrange(M):
+            dp[0][j] = points[0][j]
+        
+        for i in xrange(1, N):
+            rollingMax = float('-inf')
+            for j in xrange(M):
+                rollingMax = max(rollingMax, dp[i-1][j] - j)
+                dp[i][j] = max(dp[i][j], points[i][j] + j + rollingMax))
+            
+            rollingMax = float('-inf')
+            for j in xrange(M, -1, -1):
+                rollingMax = max(rollingMax, dp[i-1][j] + j)
+                dp[i][j] = max(dp[i][j], points[i][j] - j + rollingMax))
+
+        return max(dp[N-1])
\ No newline at end of file
diff --git a/problems/python/maximum-number-of-visible-points.py b/problems/python/maximum-number-of-visible-points.py
new file mode 100755
index 0000000..f247156
--- /dev/null
+++ b/problems/python/maximum-number-of-visible-points.py
@@ -0,0 +1,54 @@
+"""
+Time: O(NLogN), N is the number of points.
+Space: O(N)
+
+1. Get the angle of each point relative to the "location"
+2. Sort the angles
+3. Do a sliding window to angles to see what the maximum number of angles within the interval "angle"
+
+[1] For example, angles = [10, 20, 360], angle = 20 we will count 2, but actually it will be 3.
+"""
+class Solution(object):
+    def visiblePoints(self, points, angle, location):
+        def getAngle(x, y):
+            #4 axis
+            if x>0 and y==0:
+                return 0
+            elif x==0 and y>0:
+                return 90
+            elif x<0 and y==0:
+                return 180
+            elif x==0 and y<0:
+                return 270
+            
+            #4 quadrant
+            if x>0 and y>0:
+                return math.degrees(math.atan2(abs(y), abs(x)))
+            elif x<0 and y>0:
+                return 180-math.degrees(math.atan2(abs(y), abs(x)))
+            elif x<0 and y<0:
+                return 180+math.degrees(math.atan2(abs(y), abs(x)))
+            else:
+                return 360-math.degrees(math.atan2(abs(y), abs(x)))
+            
+        ans = 0
+        onLocation = 0
+        angles = []
+        
+        for x, y in points:
+            if x==location[0] and y==location[1]:
+                onLocation += 1
+            else:
+                a = getAngle(x-location[0], y-location[1])
+                angles.append(a)
+                if a<=angle: angles.append(360+a) #[1]
+        
+        angles.sort()
+        
+        i = 0
+        for j in xrange(len(angles)):
+            while angles[j]-angles[i]>angle:
+                i += 1
+            ans = max(ans, j-i+1)
+                
+        return ans+onLocation
\ No newline at end of file
diff --git a/problems/maximum-product-of-three-numbers.py b/problems/python/maximum-product-of-three-numbers.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-product-of-three-numbers.py
rename to problems/python/maximum-product-of-three-numbers.py
diff --git a/problems/maximum-product-subarray.py b/problems/python/maximum-product-subarray.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-product-subarray.py
rename to problems/python/maximum-product-subarray.py
diff --git a/problems/maximum-subarray-sum-with-one-deletion.py b/problems/python/maximum-subarray-sum-with-one-deletion.py
old mode 100644
new mode 100755
similarity index 92%
rename from problems/maximum-subarray-sum-with-one-deletion.py
rename to problems/python/maximum-subarray-sum-with-one-deletion.py
index d713751..a7b34f2
--- a/problems/maximum-subarray-sum-with-one-deletion.py
+++ b/problems/python/maximum-subarray-sum-with-one-deletion.py
@@ -23,7 +23,7 @@ def maximumSum(self, arr):
                 dp[i][1] = arr[i]
             else:
                 dp[i][0] = max(dp[i-1][0]+arr[i], arr[i])
-                dp[i][1] = max(dp[i-1][0], dp[i-1][1]+arr[i], arr[i])
+                dp[i][1] = max(dp[i-1][0], dp[i-1][1]+arr[i])
             subarrayMaxSum = max(subarrayMaxSum, dp[i][0], dp[i][1])
         
         return subarrayMaxSum
\ No newline at end of file
diff --git a/problems/maximum-subarray.py b/problems/python/maximum-subarray.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-subarray.py
rename to problems/python/maximum-subarray.py
diff --git a/problems/python/maximum-swap.py b/problems/python/maximum-swap.py
new file mode 100755
index 0000000..ffca66a
--- /dev/null
+++ b/problems/python/maximum-swap.py
@@ -0,0 +1,23 @@
+"""
+1. Generate positions. Storing the mapping between number to indices.
+2. Iterate from left, for each n1, find the largest number larger than n1 (searching from 9, 8, 7 to n1+1).
+3. Since we need to find the max output. There might be multiple the same number, we need to find the index of the rightest number.
+4. Remove n1 when it is done. Because right of the n1 should not consider it anymore.
+"""
+class Solution(object):
+    def maximumSwap(self, num):
+        numList = [int(n) for n in str(num)]
+        positions = collections.defaultdict(list)
+        for i, n in enumerate(numList): positions[n].append(i) #[1]
+        
+        i = 0
+        for i, n1 in enumerate(numList): #[2]
+            n1 = numList[i]
+            for n2 in xrange(9, n1, -1): 
+                if n2 in positions and len(positions[n2])>0:
+                    j = positions[n2][-1] #[3]
+                    numList[i], numList[j] = numList[j], numList[i]
+                    return int(''.join([str(n) for n in numList]))
+            positions[n1].pop(0) #[4]
+            
+        return num
\ No newline at end of file
diff --git a/problems/maximum-units-on-a-truck.py b/problems/python/maximum-units-on-a-truck.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/maximum-units-on-a-truck.py
rename to problems/python/maximum-units-on-a-truck.py
diff --git a/problems/median-of-two-sorted-arrays.py b/problems/python/median-of-two-sorted-arrays.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/median-of-two-sorted-arrays.py
rename to problems/python/median-of-two-sorted-arrays.py
diff --git a/problems/meeting-rooms-ii.py b/problems/python/meeting-rooms-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/meeting-rooms-ii.py
rename to problems/python/meeting-rooms-ii.py
diff --git a/problems/meeting-rooms.py b/problems/python/meeting-rooms.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/meeting-rooms.py
rename to problems/python/meeting-rooms.py
diff --git a/problems/merge-intervals.py b/problems/python/merge-intervals.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/merge-intervals.py
rename to problems/python/merge-intervals.py
diff --git a/problems/merge-k-sorted-lists.py b/problems/python/merge-k-sorted-lists.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/merge-k-sorted-lists.py
rename to problems/python/merge-k-sorted-lists.py
diff --git a/problems/merge-sorted-array.py b/problems/python/merge-sorted-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/merge-sorted-array.py
rename to problems/python/merge-sorted-array.py
diff --git a/problems/merge-two-sorted-lists.py b/problems/python/merge-two-sorted-lists.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/merge-two-sorted-lists.py
rename to problems/python/merge-two-sorted-lists.py
diff --git a/problems/min-cost-climbing-stairs.py b/problems/python/min-cost-climbing-stairs.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/min-cost-climbing-stairs.py
rename to problems/python/min-cost-climbing-stairs.py
diff --git a/problems/min-stack.py b/problems/python/min-stack.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/min-stack.py
rename to problems/python/min-stack.py
diff --git a/problems/minimize-malware-spread.py b/problems/python/minimize-malware-spread.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimize-malware-spread.py
rename to problems/python/minimize-malware-spread.py
diff --git a/problems/minimum-absolute-difference-in-bst.py b/problems/python/minimum-absolute-difference-in-bst.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-absolute-difference-in-bst.py
rename to problems/python/minimum-absolute-difference-in-bst.py
diff --git a/problems/minimum-ascii-delete-sum-for-two-strings.py b/problems/python/minimum-ascii-delete-sum-for-two-strings.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-ascii-delete-sum-for-two-strings.py
rename to problems/python/minimum-ascii-delete-sum-for-two-strings.py
diff --git a/problems/minimum-cost-to-connect-sticks.py b/problems/python/minimum-cost-to-connect-sticks.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-cost-to-connect-sticks.py
rename to problems/python/minimum-cost-to-connect-sticks.py
diff --git a/problems/minimum-cost-to-hire-k-workers.py b/problems/python/minimum-cost-to-hire-k-workers.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-cost-to-hire-k-workers.py
rename to problems/python/minimum-cost-to-hire-k-workers.py
diff --git a/problems/python/minimum-cost-to-make-at-least-one-valid-path-in-a-grid.py b/problems/python/minimum-cost-to-make-at-least-one-valid-path-in-a-grid.py
new file mode 100755
index 0000000..2188f4a
--- /dev/null
+++ b/problems/python/minimum-cost-to-make-at-least-one-valid-path-in-a-grid.py
@@ -0,0 +1,32 @@
+class Solution(object):
+    def minCost(self, grid):
+        pq = [(0, False, 0, 0, 0)]
+        visited = set()
+        M = len(grid)
+        N = len(grid[0])
+        
+        while pq:
+            cost, modified, direction, x, y = heapq.heappop(pq)
+            if x<0 or x>=M or y<0 or y>=N: continue
+            if (direction, x, y) in visited: continue
+            visited.add((direction, x, y))
+            
+            if x==M-1 and y==N-1: return cost
+            
+            if direction==0: direction = grid[x][y]
+
+            if direction==1:
+                heapq.heappush(pq, (cost, False, 0, x, y+1))
+            elif direction==2:
+                heapq.heappush(pq, (cost, False, 0, x, y-1))
+            elif direction==3:
+                heapq.heappush(pq, (cost, False, 0, x+1, y))
+            elif direction==4:
+                heapq.heappush(pq, (cost, False, 0, x-1, y))
+        
+            if not modified:    
+                for d in [1,2,3,4]:
+                    if d==grid[x][y]: continue
+                    heapq.heappush(pq, (cost+1, True, d, x, y))
+                            
+        return float('inf')
\ No newline at end of file
diff --git a/problems/python/minimum-cost-to-reach-city-with-discounts.py b/problems/python/minimum-cost-to-reach-city-with-discounts.py
new file mode 100755
index 0000000..93205f3
--- /dev/null
+++ b/problems/python/minimum-cost-to-reach-city-with-discounts.py
@@ -0,0 +1,25 @@
+class Solution(object):
+    def minimumCost(self, n, highways, discounts):
+        pq = [(0, discounts, 0)]
+        visited = set()
+        
+        adj = collections.defaultdict(list)
+        for city1, city2, toll in highways:
+            adj[city1].append((city2, toll))
+            adj[city2].append((city1, toll))
+        
+        
+        while pq:
+            toll, d, city = heapq.heappop(pq)
+            if (d, city) in visited: continue
+            visited.add((d, city))
+            
+            if city==n-1: return toll
+            
+            for nei, toll2 in adj[city]:
+                if (d, nei) not in visited:
+                    heapq.heappush(pq, (toll+toll2, d, nei))
+                if d>0 and (d-1, nei) not in visited:
+                    heapq.heappush(pq, (toll+toll2/2, d-1, nei))    
+        
+        return -1
\ No newline at end of file
diff --git a/problems/minimum-depth-of-binary-tree.py b/problems/python/minimum-depth-of-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-depth-of-binary-tree.py
rename to problems/python/minimum-depth-of-binary-tree.py
diff --git a/problems/minimum-difficulty-of-a-job-schedule.py b/problems/python/minimum-difficulty-of-a-job-schedule.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-difficulty-of-a-job-schedule.py
rename to problems/python/minimum-difficulty-of-a-job-schedule.py
diff --git a/problems/minimum-falling-path-sum-ii.py b/problems/python/minimum-falling-path-sum-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-falling-path-sum-ii.py
rename to problems/python/minimum-falling-path-sum-ii.py
diff --git a/problems/python/minimum-knight-moves.py b/problems/python/minimum-knight-moves.py
new file mode 100755
index 0000000..84cb400
--- /dev/null
+++ b/problems/python/minimum-knight-moves.py
@@ -0,0 +1,30 @@
+# Bidirectional BFS
+class Solution(object):
+    def minKnightMoves(self, x, y):
+        offsets = [(1, 2), (2, 1), (2, -1), (1, -2), (-1, -2), (-2, -1), (-2, 1), (-1, 2)]
+        q1 = collections.deque([(0, 0)])
+        q2 = collections.deque([(x, y)])
+        steps1 = {(0, 0): 0} #steps needed starting from (0, 0)
+        steps2 = {(x, y): 0} #steps needed starting from (x,y)
+        
+        while q1 and q2:
+            i1, j1 = q1.popleft()
+            if (i1, j1) in steps2: return steps1[(i1, j1)]+steps2[(i1, j1)]
+            
+            i2, j2 = q2.popleft()
+            if (i2, j2) in steps1: return steps1[(i2, j2)]+steps2[(i2, j2)]
+            
+            for ox, oy in offsets:
+                nextI1 = i1+ox
+                nextJ1 = j1+oy
+                if (nextI1, nextJ1) not in steps1:
+                    q1.append((nextI1, nextJ1))
+                    steps1[(nextI1, nextJ1)] = steps1[(i1, j1)]+1
+                
+                nextI2 = i2+ox
+                nextJ2 = j2+oy
+                if (nextI2, nextJ2) not in steps2:
+                    q2.append((nextI2, nextJ2))
+                    steps2[(nextI2, nextJ2)] = steps2[(i2, j2)]+1
+                
+        return float('inf')
\ No newline at end of file
diff --git a/problems/python/minimum-number-of-flips-to-convert-binary-matrix-to-zero-matrix.py b/problems/python/minimum-number-of-flips-to-convert-binary-matrix-to-zero-matrix.py
new file mode 100755
index 0000000..5559c80
--- /dev/null
+++ b/problems/python/minimum-number-of-flips-to-convert-binary-matrix-to-zero-matrix.py
@@ -0,0 +1,42 @@
+class Solution(object):
+    def minFlips(self, mat):
+        def flip(mat, m, n):
+            mat[m][n] = 0 if mat[m][n]==1 else 1
+            if m+1<len(mat): mat[m+1][n] = 0 if mat[m+1][n]==1 else 1
+            if n+1<len(mat[0]): mat[m][n+1] = 0 if mat[m][n+1]==1 else 1
+            if m-1>=0: mat[m-1][n] = 0 if mat[m-1][n]==1 else 1
+            if n-1>=0: mat[m][n-1] = 0 if mat[m][n-1]==1 else 1
+            
+        def check(mat, state):
+            for i, b in enumerate(state):
+                if b=='1':
+                    m = i/len(mat[0])
+                    n = i%len(mat[0])
+                    flip(mat, m, n)
+            
+            for i in xrange(len(mat)):
+                for j in xrange(len(mat[0])):
+                    if mat[i][j]==1: return False
+            
+            return True
+                    
+        
+        M = len(mat)
+        N = len(mat[0])
+        q = collections.deque(['0'*(M*N)])
+        visited = set()
+        
+        while q:
+            state = q.popleft()
+            if state in visited: continue
+            visited.add(state)
+            
+            if check([row[:] for row in mat], state): return state.count('1')
+            
+            for i in xrange(len(state)):
+                if state[i]=='1': continue
+                nextState = state[:i] + '1' +state[i+1:]
+                q.append(nextState)
+                
+        return -1
+    
\ No newline at end of file
diff --git a/problems/minimum-path-sum.py b/problems/python/minimum-path-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-path-sum.py
rename to problems/python/minimum-path-sum.py
diff --git a/problems/minimum-score-triangulation-of-polygon.py b/problems/python/minimum-score-triangulation-of-polygon.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-score-triangulation-of-polygon.py
rename to problems/python/minimum-score-triangulation-of-polygon.py
diff --git a/problems/minimum-size-subarray-sum.py b/problems/python/minimum-size-subarray-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-size-subarray-sum.py
rename to problems/python/minimum-size-subarray-sum.py
diff --git a/problems/minimum-swaps-to-group-all-1s-together.py b/problems/python/minimum-swaps-to-group-all-1s-together.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-swaps-to-group-all-1s-together.py
rename to problems/python/minimum-swaps-to-group-all-1s-together.py
diff --git a/problems/minimum-swaps-to-make-sequences-increasing.py b/problems/python/minimum-swaps-to-make-sequences-increasing.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-swaps-to-make-sequences-increasing.py
rename to problems/python/minimum-swaps-to-make-sequences-increasing.py
diff --git a/problems/python/minimum-time-difference.py b/problems/python/minimum-time-difference.py
new file mode 100755
index 0000000..1645692
--- /dev/null
+++ b/problems/python/minimum-time-difference.py
@@ -0,0 +1,32 @@
+"""
+Time: O(1440)
+Space: O(1440)
+"""
+class Solution(object):
+    def findMinDifference(self, timeStrings):
+        def timeStringToMinutes(timeString):
+            time = timeString.split(':')
+            h = int(time[0])
+            m = int(time[1])
+            return h*60+m
+        
+        ans = float('inf')
+        minTime = float('inf')
+        maxTime = float('-inf')
+        timeSet = set()
+        for timeString in timeStrings:
+            t = timeStringToMinutes(timeString)
+            if t in timeSet: return 0
+            minTime = min(minTime, t)
+            maxTime = max(maxTime, t)
+            timeSet.add(t)
+        
+        
+        prev = None
+        for t in xrange(minTime, maxTime+1):
+            if t not in timeSet: continue
+            if prev!=None: ans = min(ans, t-prev)
+            prev = t
+        
+        ans = min(ans, 1440+minTime-maxTime) #compare minTime and maxTime
+        return ans
\ No newline at end of file
diff --git a/problems/minimum-window-substring.py b/problems/python/minimum-window-substring.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/minimum-window-substring.py
rename to problems/python/minimum-window-substring.py
diff --git a/problems/python/minimum-xor-sum-of-two-arrays.py b/problems/python/minimum-xor-sum-of-two-arrays.py
new file mode 100755
index 0000000..7c3a195
--- /dev/null
+++ b/problems/python/minimum-xor-sum-of-two-arrays.py
@@ -0,0 +1,24 @@
+"""
+state[i] := nums1[i] has been matched.
+"""
+class Solution(object):
+    def minimumXORSum(self, nums1, nums2):
+        N = len(nums1)
+        
+        pq = [(0, '0'*N)]
+        visited = set()
+        while pq:
+            s, state = heapq.heappop(pq)
+            if state in visited: continue
+            visited.add(state)
+            
+            j = state.count('1')
+            if j==N: return s
+            
+            for i in xrange(N):
+                if state[i]=='1': continue
+                nextState = state[:i]+'1'+state[i+1:]
+                if nextState in visited: continue
+                heapq.heappush(pq, ((s+(nums1[i]^nums2[j-1]), nextState)))
+        
+        return float('inf')
\ No newline at end of file
diff --git a/problems/missing-number.py b/problems/python/missing-number.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/missing-number.py
rename to problems/python/missing-number.py
diff --git a/problems/most-frequent-subtree-sum.py b/problems/python/most-frequent-subtree-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/most-frequent-subtree-sum.py
rename to problems/python/most-frequent-subtree-sum.py
diff --git a/problems/most-stones-removed-with-same-row-or-column.py b/problems/python/most-stones-removed-with-same-row-or-column.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/most-stones-removed-with-same-row-or-column.py
rename to problems/python/most-stones-removed-with-same-row-or-column.py
diff --git a/problems/move-zeroes.py b/problems/python/move-zeroes.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/move-zeroes.py
rename to problems/python/move-zeroes.py
diff --git a/problems/moving-average-from-data-stream.py b/problems/python/moving-average-from-data-stream.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/moving-average-from-data-stream.py
rename to problems/python/moving-average-from-data-stream.py
diff --git a/problems/my-calendar-ii.py b/problems/python/my-calendar-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/my-calendar-ii.py
rename to problems/python/my-calendar-ii.py
diff --git a/problems/n-ary-tree-level-order-traversal.py b/problems/python/n-ary-tree-level-order-traversal.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/n-ary-tree-level-order-traversal.py
rename to problems/python/n-ary-tree-level-order-traversal.py
diff --git a/problems/n-ary-tree-postorder-traversal.py b/problems/python/n-ary-tree-postorder-traversal.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/n-ary-tree-postorder-traversal.py
rename to problems/python/n-ary-tree-postorder-traversal.py
diff --git a/problems/n-ary-tree-preorder-traversal.py b/problems/python/n-ary-tree-preorder-traversal.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/n-ary-tree-preorder-traversal.py
rename to problems/python/n-ary-tree-preorder-traversal.py
diff --git a/problems/nested-list-weight-sum.py b/problems/python/nested-list-weight-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/nested-list-weight-sum.py
rename to problems/python/nested-list-weight-sum.py
diff --git a/problems/network-delay-time.py b/problems/python/network-delay-time.py
old mode 100644
new mode 100755
similarity index 58%
rename from problems/network-delay-time.py
rename to problems/python/network-delay-time.py
index 8098a31..aa67bb0
--- a/problems/network-delay-time.py
+++ b/problems/python/network-delay-time.py
@@ -118,12 +118,96 @@ def networkDelayTime(self, times, n, k):
             
         
 
+"""
+Dijkstra BFS+Priority Queue Implementation
+Time: O(ELogE), E is the edges count in the graph.
+"""
+class Solution(object):
+    def networkDelayTime(self, times, N, K):
+        ans = -1 #max time from K to any
+        visited = set()
+        pq = [(0, K)] #[(node's distance to K, node)]
+        
+        #construct adjacency list
+        adj = collections.defaultdict(list)
+        for u, v, w in times:
+            adj[u].append((w, v))
+            
+        while pq:
+            dis, node = heapq.heappop(pq)
+            if node in visited: continue
+            visited.add(node)
+            
+            ans = max(ans, dis)
+            
+            for d2, nei in adj[node]:
+                heapq.heappush(pq, (d2+dis, nei))
+        
+        return ans if len(visited)==N else -1
 
 
 
+"""
+Dijkstra Normal Implementation
+Time: O(N^2), N is the nodes count in the graph.
+"""
+class Solution(object):
+    def networkDelayTime(self, times, N, K):
+        #construct dis. dis[n] := node n distance to K
+        dis = {}
+        for n in xrange(1, N+1):
+            dis[n] = float('inf')
+        dis[K] = 0
+        
+        visited = set()
+        
+        #construct adjacency list
+        adj = collections.defaultdict(list)
+        for u, v, w in times:
+            adj[u].append((w, v))
+        
 
+        while len(visited)<N:
+            #find the nearest node to K, lets call it minDisNode
+            minDisNode = None
+            minDis = float('inf')
+            
+            for n in xrange(1, N+1):
+                if n not in visited and dis[n]<minDis:
+                    minDisNode = n
+                    minDis = dis[n]
+            
+            #update minDisNode neighbor's distance to K    
+            if minDis==float('inf'): break
+            for d2, nei in adj[minDisNode]:
+                dis[nei] = min(dis[nei], d2+minDis)
+            
+            #store the node we alreay determined the distance to K in visited
+            visited.add(minDisNode)
+        
+        return max(dis.values()) if len(visited)==N else -1
 
 
-
-
-
+"""
+Floyd
+Time: O(N^3), N is the nodes count in the graph.
+Note that Floyed can calculate the shortest distance between any two nodes.
+Also, it can handle negative values on the edge.
+"""
+class Solution(object):
+    def networkDelayTime(self, times, N, K):
+        #dp[i-1][j-1] := distance from i to j
+        dp = [[float('inf') for i in xrange(1, N+1)] for j in xrange(1, N+1)]
+        
+        for i in xrange(1, N+1):
+            dp[i-1][i-1] = 0
+        for u, v, w in times:
+            dp[u-1][v-1] = w
+        
+        for k in xrange(1, N+1):
+            for i in xrange(1, N+1):
+                for j in xrange(1, N+1):
+                    dp[i-1][j-1] = min(dp[i-1][j-1], dp[i-1][k-1]+dp[k-1][j-1])
+        
+        ans = max(dp[K-1])
+        return ans if ans!=float('inf') else -1
\ No newline at end of file
diff --git a/problems/next-closest-time.py b/problems/python/next-closest-time.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/next-closest-time.py
rename to problems/python/next-closest-time.py
diff --git a/problems/python/next-permutation.py b/problems/python/next-permutation.py
new file mode 100755
index 0000000..0a816ee
--- /dev/null
+++ b/problems/python/next-permutation.py
@@ -0,0 +1,61 @@
+"""
+This answer is the python version of the offical answer.
+
+Time: O(N)
+Space: O(1)
+"""
+class Solution(object):
+    def nextPermutation(self, nums):
+        def reverse(start):
+            end = len(nums)-1
+            
+            while start<end:
+                swap(start, end)
+                start += 1
+                end -= 1
+        
+        def swap(i, j):
+            nums[i], nums[j] = nums[j], nums[i]
+        
+        
+        i = len(nums)-2
+        
+        while i>=0 and nums[i+1]<=nums[i]:
+            i -= 1
+        
+        if i>=0:
+            j = len(nums)-1
+            while nums[j]<=nums[i]: j -= 1
+            swap(i, j)
+        
+        reverse(i+1)
+        return nums
+
+"""
+Next Permutation means find the next (slightly) larger number using nums.
+
+1. Iterate from right, find the first num that is smaller. That's the one we are going to swap. => nums[i]
+2. From the nums right to nums[i], find the smallest num that is larger than nums[i] => nums[j]
+Since the right of the i must be an increasing sequence (looking from right), the first one that larger than nums[i] is the smallest one that is larger than nums[i]
+3. Swap nums[i] and nums[j]
+4. sort nums[i+1:] it will be the smallest permutaion.
+5. Note that when an list is in increasing order looking from right, we can use `reverse` to sort it.
+"""
+class Solution(object):
+    def nextPermutation(self, nums):
+        def reverse(nums, l, r):
+            while l<=r:
+                nums[l], nums[r] = nums[r], nums[l]
+                l += 1
+                r -= 1
+            
+        i = len(nums)-2
+        while i>=0 and nums[i]>=nums[i+1]: i -= 1 #[1]
+        
+        if i==-1:
+            return reverse(nums, 0, len(nums)-1) #nums is the largest permutation, sort nums
+        else:
+            j = len(nums)-1
+            while j>i and nums[j]<=nums[i]: j -= 1 #[2]
+            nums[i], nums[j] = nums[j], nums[i] #[3]
+            reverse(nums, i+1, len(nums)-1) #[4]
\ No newline at end of file
diff --git a/problems/number-complement.py b/problems/python/number-complement.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-complement.py
rename to problems/python/number-complement.py
diff --git a/problems/number-of-connected-components-in-an-undirected-graph.py b/problems/python/number-of-connected-components-in-an-undirected-graph.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-connected-components-in-an-undirected-graph.py
rename to problems/python/number-of-connected-components-in-an-undirected-graph.py
diff --git a/problems/number-of-islands-ii.py b/problems/python/number-of-islands-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-islands-ii.py
rename to problems/python/number-of-islands-ii.py
diff --git a/problems/number-of-islands.py b/problems/python/number-of-islands.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-islands.py
rename to problems/python/number-of-islands.py
diff --git a/problems/number-of-longest-increasing-subsequence.py b/problems/python/number-of-longest-increasing-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-longest-increasing-subsequence.py
rename to problems/python/number-of-longest-increasing-subsequence.py
diff --git a/problems/python/number-of-matching-subsequences.py b/problems/python/number-of-matching-subsequences.py
new file mode 100755
index 0000000..fd97676
--- /dev/null
+++ b/problems/python/number-of-matching-subsequences.py
@@ -0,0 +1,25 @@
+"""
+Time: O(WL x LogS), W is the length of words, L is the length of word.
+LogS is the time for binary search and S should be the count of repetitive words, we can assume it is Log(S/26) ~= LogS.
+
+Space: O(S)
+"""
+class Solution(object):
+    def numMatchingSubseq(self, s, words):
+        def match(position, word):
+            prev = -1
+            for c in word:
+                if c not in position: return False
+                i = bisect.bisect_left(position[c], prev+1)
+                if i==len(position[c]): return False
+                prev = position[c][i]
+            return True
+        
+        position = collections.defaultdict(list)
+        count = 0
+        for i, c in enumerate(s):
+            position[c].append(i)
+        
+        for word in words:
+            if match(position, word): count += 1
+        return count
\ No newline at end of file
diff --git a/problems/number-of-provinces.py b/problems/python/number-of-provinces.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-provinces.py
rename to problems/python/number-of-provinces.py
diff --git a/problems/number-of-recent-calls.py b/problems/python/number-of-recent-calls.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-recent-calls.py
rename to problems/python/number-of-recent-calls.py
diff --git a/problems/number-of-squareful-arrays.py b/problems/python/number-of-squareful-arrays.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/number-of-squareful-arrays.py
rename to problems/python/number-of-squareful-arrays.py
diff --git a/problems/python/number-of-substrings-containing-all-thre.py b/problems/python/number-of-substrings-containing-all-thre.py
new file mode 100755
index 0000000..66d43cc
--- /dev/null
+++ b/problems/python/number-of-substrings-containing-all-thre.py
@@ -0,0 +1,22 @@
+class Solution(object):
+    def numberOfSubstrings(self, s):
+        #number of subarrays that at most have k unique char
+        def atMost(k):
+            counter = collections.Counter()
+            uniqueCount = 0
+            ans = 0
+            i = 0
+            
+            for j, c in enumerate(s):
+                counter[c] += 1
+                if counter[c]==1: uniqueCount += 1
+
+                while uniqueCount>k:
+                    counter[s[i]] -= 1
+                    if counter[s[i]]==0: uniqueCount-= 1
+                    i += 1
+                ans += j-i+1
+            return ans
+        
+        n = len(s)
+        return atMost(3) - atMost(2)
\ No newline at end of file
diff --git a/problems/python/number-of-ways-to-arrive-at-destination.py b/problems/python/number-of-ways-to-arrive-at-destination.py
new file mode 100755
index 0000000..3231254
--- /dev/null
+++ b/problems/python/number-of-ways-to-arrive-at-destination.py
@@ -0,0 +1,33 @@
+class Solution(object):
+    def countPaths(self, n, roads):
+        def countWaysToReach(node):
+            if node==0: return 1
+            if node in history: return history[node]
+            c = 0
+            for nei, t in adj[node]:
+                if nei in times and times[nei]+t==times[node]:
+                    c += countWaysToReach(nei)
+            history[node] = c
+            return c
+        
+        history = {} #cache for countWaysToReach()
+        times = {} #min times to reach node n-1
+        pq = [(0, 0)]
+        
+        adj = collections.defaultdict(list)
+        for u, v, t in roads:
+            adj[u].append((v, t))
+            adj[v].append((u, t))
+        
+        while pq:
+            t, node = heapq.heappop(pq)
+            if node in times: continue
+            times[node] = t
+            
+            if node==n-1: break
+            
+            for nei, t2 in adj[node]:
+                if nei in times: continue
+                heapq.heappush(pq, (t+t2, nei))
+        
+        return countWaysToReach(n-1)%(10**9 + 7)
\ No newline at end of file
diff --git a/problems/odd-even-jump.py b/problems/python/odd-even-jump.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/odd-even-jump.py
rename to problems/python/odd-even-jump.py
diff --git a/problems/ones-and-zeroes.py b/problems/python/ones-and-zeroes.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/ones-and-zeroes.py
rename to problems/python/ones-and-zeroes.py
diff --git a/problems/open-the-lock.py b/problems/python/open-the-lock.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/open-the-lock.py
rename to problems/python/open-the-lock.py
diff --git a/problems/out-of-boundary-paths.py b/problems/python/out-of-boundary-paths.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/out-of-boundary-paths.py
rename to problems/python/out-of-boundary-paths.py
diff --git a/problems/pacific-atlantic-water-flow.py b/problems/python/pacific-atlantic-water-flow.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/pacific-atlantic-water-flow.py
rename to problems/python/pacific-atlantic-water-flow.py
diff --git a/problems/pairs-of-songs-with-total-durations-divisible-by-60.py b/problems/python/pairs-of-songs-with-total-durations-divisible-by-60.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/pairs-of-songs-with-total-durations-divisible-by-60.py
rename to problems/python/pairs-of-songs-with-total-durations-divisible-by-60.py
diff --git a/problems/palindrome-number.py b/problems/python/palindrome-number.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/palindrome-number.py
rename to problems/python/palindrome-number.py
diff --git a/problems/palindrome-pairs.py b/problems/python/palindrome-pairs.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/palindrome-pairs.py
rename to problems/python/palindrome-pairs.py
diff --git a/problems/palindrome-partitioning-iii.py b/problems/python/palindrome-partitioning-iii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/palindrome-partitioning-iii.py
rename to problems/python/palindrome-partitioning-iii.py
diff --git a/problems/palindrome-partitioning.py b/problems/python/palindrome-partitioning.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/palindrome-partitioning.py
rename to problems/python/palindrome-partitioning.py
diff --git a/problems/palindromic-substrings.py b/problems/python/palindromic-substrings.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/palindromic-substrings.py
rename to problems/python/palindromic-substrings.py
diff --git a/problems/partition-array-for-maximum-sum.py b/problems/python/partition-array-for-maximum-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/partition-array-for-maximum-sum.py
rename to problems/python/partition-array-for-maximum-sum.py
diff --git a/problems/partition-labels.py b/problems/python/partition-labels.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/partition-labels.py
rename to problems/python/partition-labels.py
diff --git a/problems/partition-to-k-equal-sum-subsets.py b/problems/python/partition-to-k-equal-sum-subsets.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/partition-to-k-equal-sum-subsets.py
rename to problems/python/partition-to-k-equal-sum-subsets.py
diff --git a/problems/path-sum-ii.py b/problems/python/path-sum-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/path-sum-ii.py
rename to problems/python/path-sum-ii.py
diff --git a/problems/path-sum-iii.py b/problems/python/path-sum-iii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/path-sum-iii.py
rename to problems/python/path-sum-iii.py
diff --git a/problems/path-sum.py b/problems/python/path-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/path-sum.py
rename to problems/python/path-sum.py
diff --git a/problems/python/path-with-maximum-probability.py b/problems/python/path-with-maximum-probability.py
new file mode 100755
index 0000000..db1b1d4
--- /dev/null
+++ b/problems/python/path-with-maximum-probability.py
@@ -0,0 +1,24 @@
+class Solution(object):
+    def maxProbability(self, n, edges, succProb, start, end):
+        pq = [(-1, start)]
+        visited = set()
+        adj = collections.defaultdict(list)
+        for i in xrange(len(edges)):
+            a, b = edges[i]
+            p = succProb[i]
+            adj[a].append((b, p))
+            adj[b].append((a, p))
+        
+        while pq:
+            p, node = heapq.heappop(pq)
+            p = p*-1
+            if node in visited: continue
+            visited.add(node)
+            
+            if node==end: return p
+            
+            for nei, p2 in adj[node]:
+                if nei in visited: continue
+                heapq.heappush(pq, (-1*p*p2, nei))
+        
+        return 0
\ No newline at end of file
diff --git a/problems/peak-index-in-a-mountain-array.PY b/problems/python/peak-index-in-a-mountain-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/peak-index-in-a-mountain-array.PY
rename to problems/python/peak-index-in-a-mountain-array.py
diff --git a/problems/perfect-squares.py b/problems/python/perfect-squares.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/perfect-squares.py
rename to problems/python/perfect-squares.py
diff --git a/problems/permutation-in-string.py b/problems/python/permutation-in-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/permutation-in-string.py
rename to problems/python/permutation-in-string.py
diff --git a/problems/python/permutation-sequence.py b/problems/python/permutation-sequence.py
new file mode 100755
index 0000000..f7e5be2
--- /dev/null
+++ b/problems/python/permutation-sequence.py
@@ -0,0 +1,14 @@
+class Solution(object):
+    def getPermutation(self, N, K):
+        K = K-1 #make it 0-index
+        nums = range(1, N+1)
+        ans = ''
+        
+        while N>0:
+            a = K/math.factorial(N-1)
+            ans += str(nums[a])
+            nums.pop(a)
+            
+            K -= math.factorial(N-1)*(a+1)
+            N -= 1
+        return ans
\ No newline at end of file
diff --git a/problems/permutations-ii.py b/problems/python/permutations-ii.py
old mode 100644
new mode 100755
similarity index 71%
rename from problems/permutations-ii.py
rename to problems/python/permutations-ii.py
index e4aa3c2..a6c9ff7
--- a/problems/permutations-ii.py
+++ b/problems/python/permutations-ii.py
@@ -53,3 +53,28 @@ def helper(path):
         counter = collections.Counter(nums)
         helper([])
         return ans
+
+
+"""
+差板法
+"""
+class Solution(object):
+    def permuteUnique(self, nums):
+        if not nums: return []
+        
+        permutations = collections.deque([[nums[0]]])
+        
+        for i in xrange(1, len(nums)):
+            num = nums[i]
+            l = len(permutations)    
+            
+            while l:
+                permutation = permutations.popleft()
+                for j in xrange(len(permutation)+1):
+                    if 0<j and num==permutation[j-1]: break
+                    newPermutaion = permutation[:]
+                    newPermutaion.insert(j, num)
+                    permutations.append(newPermutaion)
+                l -= 1
+        
+        return permutations
\ No newline at end of file
diff --git a/problems/permutations.py b/problems/python/permutations.py
old mode 100644
new mode 100755
similarity index 79%
rename from problems/permutations.py
rename to problems/python/permutations.py
index bc27184..eb40d3a
--- a/problems/permutations.py
+++ b/problems/python/permutations.py
@@ -73,4 +73,28 @@ def helper(i):
         N = len(nums)
         ans = []
         helper(0)
-        return ans
\ No newline at end of file
+        return ans
+
+
+"""
+差板法
+"""
+class Solution(object):
+    def permute(self, nums):
+        if not nums: return []
+        
+        permutations = collections.deque([[nums[0]]])
+        
+        for i in xrange(1, len(nums)):
+            num = nums[i]
+            l = len(permutations)    
+            
+            while l:
+                permutation = permutations.popleft()
+                for j in xrange(len(permutation)+1):
+                    newPermutaion = permutation[:]
+                    newPermutaion.insert(j, num)
+                    permutations.append(newPermutaion)
+                l -= 1
+        
+        return permutations
\ No newline at end of file
diff --git a/problems/populating-next-right-pointers-in-each-node-ii.py b/problems/python/populating-next-right-pointers-in-each-node-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/populating-next-right-pointers-in-each-node-ii.py
rename to problems/python/populating-next-right-pointers-in-each-node-ii.py
diff --git a/problems/populating-next-right-pointers-in-each-node.py b/problems/python/populating-next-right-pointers-in-each-node.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/populating-next-right-pointers-in-each-node.py
rename to problems/python/populating-next-right-pointers-in-each-node.py
diff --git a/problems/python/powx-n.py b/problems/python/powx-n.py
new file mode 100755
index 0000000..d0fac2b
--- /dev/null
+++ b/problems/python/powx-n.py
@@ -0,0 +1,12 @@
+class Solution(object):
+    def myPow(self, x, n):
+        if n==0:
+            return 1
+        elif n<0:
+            return 1/self.myPow(x, -n)
+        elif n%2>0:
+            half = self.myPow(x, n-1)
+            return x*half
+        elif n%2==0:
+            half = self.myPow(x, n/2)
+            return half*half
\ No newline at end of file
diff --git a/problems/product-of-array-except-self.py b/problems/python/product-of-array-except-self.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/product-of-array-except-self.py
rename to problems/python/product-of-array-except-self.py
diff --git a/problems/python/profitable-schemes.py b/problems/python/profitable-schemes.py
new file mode 100755
index 0000000..2c16d85
--- /dev/null
+++ b/problems/python/profitable-schemes.py
@@ -0,0 +1,45 @@
+"""
+TLE
+dp[i][n][p] := considering profit[:i], what is the number of ways produce profit p with n people.
+"""
+class Solution(object):
+    def profitableSchemes(self, maxMember, minProfit, group, profit):
+        P = sum(profit)
+        N = sum(group)
+        dp = [[[0 for _ in xrange(P+1)] for _ in xrange(N+1)] for _ in xrange(len(profit)+1)]
+        dp[0][0][0] = 1
+        
+        count = 0
+        for i in xrange(1, len(profit)+1):
+            for n in xrange(N+1):
+                for p in xrange(P+1):
+                    dp[i][n][p] = (dp[i-1][n-group[i-1]][p-profit[i-1]] if p-profit[i-1]>=0 and n-group[i-1]>=0 else 0) + dp[i-1][n][p]
+                    if i==len(profit) and p>=minProfit and n<=maxMember: count += dp[i][n][p]
+        return count
+
+
+
+"""
+dp[i][g][p] := consider only crime[:i] the scheme that can generate profit p using man power g.
+"""
+class Solution(object):
+    def profitableSchemes(self, n, minProfit, group, profit):
+        N = len(profit)
+        
+        dp = [[[0]*(n+2) for _ in xrange(minProfit+1)] for _ in xrange(N+1)]
+        dp[0][0][0] = 1
+        
+        for i in xrange(1, N+1):
+            for p in xrange(minProfit+1):
+                for g in xrange(n+1):
+                    pi = profit[i-1]
+                    gi = group[i-1]
+                    
+                    #considerting last round using p and g
+                    dp[i][p][g] += dp[i-1][p][g]
+                    dp[i][min(pi+p, minProfit)][min(gi+g, n+1)] += dp[i-1][p][g]
+        
+        ans = 0
+        for g in xrange(n+1):
+            ans += dp[N][minProfit][g]
+        return ans % (10**9 + 7)
\ No newline at end of file
diff --git a/problems/queue-reconstruction-by-height.py b/problems/python/queue-reconstruction-by-height.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/queue-reconstruction-by-height.py
rename to problems/python/queue-reconstruction-by-height.py
diff --git a/problems/python/random-pick-with-weight.py b/problems/python/random-pick-with-weight.py
new file mode 100755
index 0000000..9c88bd0
--- /dev/null
+++ b/problems/python/random-pick-with-weight.py
@@ -0,0 +1,29 @@
+"""
+For W = [1,3,2,4] (total 10, 1+2+3+4)
+Think of a line _,___,__,____ ([1,4,6,10])
+Now we randomly throw a ball on the line the probability to land on the first section will be 1/10.
+the second section, 3/10.
+the third section, 2/10.
+the forth section, 4/10.
+
+Above is equivilant to we randomly pick a number [0~10) and see which section it is in.
+And since the cumulative probability in the "line" is strictly increasing, we can use binary search.
+
+Time: O(LogN), N is the number of W.
+Space: O(N).
+"""
+from random import randrange
+
+class Solution(object):
+
+    def __init__(self, W):
+        self.line = [] #cumulative probability distribution
+        self.total = 0
+        
+        for w in W:
+            self.total += w
+            self.line.append(self.total)
+
+    def pickIndex(self):
+        rand = random.randrange(self.total)
+        return bisect.bisect(self.line, rand)
\ No newline at end of file
diff --git a/problems/range-addition.py b/problems/python/range-addition.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/range-addition.py
rename to problems/python/range-addition.py
diff --git a/problems/python/range-sum-of-bst.py b/problems/python/range-sum-of-bst.py
new file mode 100755
index 0000000..0ca07ed
--- /dev/null
+++ b/problems/python/range-sum-of-bst.py
@@ -0,0 +1,11 @@
+class Solution(object):
+    def rangeSumBST(self, root, low, high):
+        def helper(node, low, high):
+            total = 0
+            if not node: return total
+            if low<=node.val<=high: total += node.val
+            if node.val<=high: total += helper(node.right, low, high)
+            if node.val>=low: total += helper(node.left, low, high)
+            return total
+        
+        return helper(root, low, high)
\ No newline at end of file
diff --git a/problems/range-sum-query-immutable.py b/problems/python/range-sum-query-immutable.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/range-sum-query-immutable.py
rename to problems/python/range-sum-query-immutable.py
diff --git a/problems/range-sum-query-mutable.py b/problems/python/range-sum-query-mutable.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/range-sum-query-mutable.py
rename to problems/python/range-sum-query-mutable.py
diff --git a/problems/rearrange-string-k-distance-apart.py b/problems/python/rearrange-string-k-distance-apart.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/rearrange-string-k-distance-apart.py
rename to problems/python/rearrange-string-k-distance-apart.py
diff --git a/problems/reconstruct-itinerary.py b/problems/python/reconstruct-itinerary.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/reconstruct-itinerary.py
rename to problems/python/reconstruct-itinerary.py
diff --git a/problems/recover-binary-search-tree.py b/problems/python/recover-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/recover-binary-search-tree.py
rename to problems/python/recover-binary-search-tree.py
diff --git a/problems/redundant-connection.py b/problems/python/redundant-connection.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/redundant-connection.py
rename to problems/python/redundant-connection.py
diff --git a/problems/python/remove-all-adjacent-duplicates-in-string.py b/problems/python/remove-all-adjacent-duplicates-in-string.py
new file mode 100755
index 0000000..f4bb906
--- /dev/null
+++ b/problems/python/remove-all-adjacent-duplicates-in-string.py
@@ -0,0 +1,12 @@
+class Solution(object):
+    def removeDuplicates(self, s):
+        stack = []
+        i = 0
+        while i<len(s):
+            if stack and stack[-1]==s[i]:
+                stack.pop()    
+            else:
+                stack.append(s[i])
+            i += 1
+        
+        return ''.join(stack)
\ No newline at end of file
diff --git a/problems/python/remove-all-ones-with-row-and-column-flips.py b/problems/python/remove-all-ones-with-row-and-column-flips.py
new file mode 100755
index 0000000..69093cc
--- /dev/null
+++ b/problems/python/remove-all-ones-with-row-and-column-flips.py
@@ -0,0 +1,28 @@
+"""
+Not sure how to scientifically prove this.
+In order to turn all the 1 to 0, every row need to have "the same pattern". Or it is imposible.
+This same pattern is means they are 1. identical 2. entirely different.
+For example 101 and 101, 101 and 010.
+110011 and 110011. 110011 and 001100.
+
+Time: O(N), N is the number of element in the grid.
+Space: O(N)
+"""
+class Solution(object):
+    def removeOnes(self, grid):
+        if not grid: return True
+        rowStrings = set()
+        
+        for row in grid:
+            rowStrings.add(''.join((str(e) for e in row)))
+        
+        if len(rowStrings)>2: return False
+        if len(rowStrings)==1: return True
+        
+        s1 = rowStrings.pop()
+        s2 = rowStrings.pop()
+        
+        for i in xrange(len(s1)):
+            if (s1[i]=='0' and s2[i]=='1') or (s1[i]=='1' and s2[i]=='0'): continue
+            return False
+        return True
\ No newline at end of file
diff --git a/problems/remove-duplicates-from-sorted-array-ii.py b/problems/python/remove-duplicates-from-sorted-array-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/remove-duplicates-from-sorted-array-ii.py
rename to problems/python/remove-duplicates-from-sorted-array-ii.py
diff --git a/problems/remove-duplicates-from-sorted-array.py b/problems/python/remove-duplicates-from-sorted-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/remove-duplicates-from-sorted-array.py
rename to problems/python/remove-duplicates-from-sorted-array.py
diff --git a/problems/remove-duplicates-from-sorted-list.py b/problems/python/remove-duplicates-from-sorted-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/remove-duplicates-from-sorted-list.py
rename to problems/python/remove-duplicates-from-sorted-list.py
diff --git a/problems/remove-element.py b/problems/python/remove-element.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/remove-element.py
rename to problems/python/remove-element.py
diff --git a/problems/python/remove-invalid-parentheses.py b/problems/python/remove-invalid-parentheses.py
new file mode 100755
index 0000000..0cb7695
--- /dev/null
+++ b/problems/python/remove-invalid-parentheses.py
@@ -0,0 +1,38 @@
+class Solution(object):
+    def removeInvalidParentheses(self, s):
+        def dfs(s, curr, i, count):
+            if count<0: return
+            if len(curr)>self.maxLen: return
+            
+            if i>=len(s):
+                if len(curr)==self.maxLen and count==0:
+                    self.ans.append(curr)
+                return
+            
+            if s[i]!='(' and s[i]!=')':
+                dfs(s, curr+s[i], i+1, count)
+            elif not curr or s[i]!=curr[-1]:
+                dfs(s, curr+s[i], i+1, count + (1 if s[i]=='(' else -1))
+                dfs(s, curr, i+1, count)
+            elif s[i]==curr[-1]:
+                dfs(s, curr+s[i], i+1, count + (1 if s[i]=='(' else -1))
+                
+        def getMaxLen(s):
+            openCount = removeCount = 0
+            for c in s:
+                if c=='(':
+                    openCount += 1
+                elif c==')':
+                    openCount -= 1
+                
+                if openCount<0:
+                    removeCount += abs(openCount)
+                    openCount = 0
+            removeCount += openCount   
+            return len(s)-removeCount
+        
+        self.ans = []
+        self.maxLen = getMaxLen(s)
+        
+        dfs(s, "", 0, 0)
+        return self.ans
\ No newline at end of file
diff --git a/problems/remove-linked-list-elements.py b/problems/python/remove-linked-list-elements.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/remove-linked-list-elements.py
rename to problems/python/remove-linked-list-elements.py
diff --git a/problems/remove-nth-node-from-end-of-list.py b/problems/python/remove-nth-node-from-end-of-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/remove-nth-node-from-end-of-list.py
rename to problems/python/remove-nth-node-from-end-of-list.py
diff --git a/problems/reorder-list.py b/problems/python/reorder-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/reorder-list.py
rename to problems/python/reorder-list.py
diff --git a/problems/repeated-string-match.py b/problems/python/repeated-string-match.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/repeated-string-match.py
rename to problems/python/repeated-string-match.py
diff --git a/problems/python/replace-the-substring-for-balanced-string.py b/problems/python/replace-the-substring-for-balanced-string.py
new file mode 100755
index 0000000..0ce1250
--- /dev/null
+++ b/problems/python/replace-the-substring-for-balanced-string.py
@@ -0,0 +1,15 @@
+class Solution(object):
+    def balancedString(self, s):
+        n = len(s)
+        counter = collections.Counter(s)
+        i = 0
+        ans = n
+        
+        for j, c in enumerate(s):
+            counter[c] -= 1
+            
+            while i<n and all(counter[c]<=n/4 for c in 'QEWR'):
+                counter[s[i]] += 1
+                ans = min(ans, j-i+1)
+                i += 1
+        return ans
\ No newline at end of file
diff --git a/problems/restore-ip-addresses.py b/problems/python/restore-ip-addresses.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/restore-ip-addresses.py
rename to problems/python/restore-ip-addresses.py
diff --git a/problems/reverse-integer.py b/problems/python/reverse-integer.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/reverse-integer.py
rename to problems/python/reverse-integer.py
diff --git a/problems/reverse-linked-list.py b/problems/python/reverse-linked-list.py
old mode 100644
new mode 100755
similarity index 73%
rename from problems/reverse-linked-list.py
rename to problems/python/reverse-linked-list.py
index 46eedea..9b60a26
--- a/problems/reverse-linked-list.py
+++ b/problems/python/reverse-linked-list.py
@@ -43,3 +43,19 @@ def reverseList(self, node):
             if not next_node: return node
             pre = node
             node = next_node
+
+
+# Definition for singly-linked list.
+# class ListNode(object):
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution(object):
+    def reverseList(self, head):
+        if not head or not head.next: return head
+        
+        reversedHead = self.reverseList(head.next)
+        head.next.next = head
+        head.next = None
+        return reversedHead
+        
\ No newline at end of file
diff --git a/problems/reverse-string.py b/problems/python/reverse-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/reverse-string.py
rename to problems/python/reverse-string.py
diff --git a/problems/reverse-vowels-of-a-string.py b/problems/python/reverse-vowels-of-a-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/reverse-vowels-of-a-string.py
rename to problems/python/reverse-vowels-of-a-string.py
diff --git a/problems/reverse-words-in-a-string.py b/problems/python/reverse-words-in-a-string.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/reverse-words-in-a-string.py
rename to problems/python/reverse-words-in-a-string.py
diff --git a/problems/robot-bounded-in-circle.py b/problems/python/robot-bounded-in-circle.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/robot-bounded-in-circle.py
rename to problems/python/robot-bounded-in-circle.py
diff --git a/problems/roman-to-integer.py b/problems/python/roman-to-integer.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/roman-to-integer.py
rename to problems/python/roman-to-integer.py
diff --git a/problems/rotate-array.py b/problems/python/rotate-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/rotate-array.py
rename to problems/python/rotate-array.py
diff --git a/problems/rotate-image.py b/problems/python/rotate-image.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/rotate-image.py
rename to problems/python/rotate-image.py
diff --git a/problems/russian-doll-envelopes.py b/problems/python/russian-doll-envelopes.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/russian-doll-envelopes.py
rename to problems/python/russian-doll-envelopes.py
diff --git a/problems/same-tree.py b/problems/python/same-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/same-tree.py
rename to problems/python/same-tree.py
diff --git a/problems/satisfiability-of-equality-equations.py b/problems/python/satisfiability-of-equality-equations.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/satisfiability-of-equality-equations.py
rename to problems/python/satisfiability-of-equality-equations.py
diff --git a/problems/score-of-parentheses.py b/problems/python/score-of-parentheses.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/score-of-parentheses.py
rename to problems/python/score-of-parentheses.py
diff --git a/problems/search-a-2d-matrix.py b/problems/python/search-a-2d-matrix.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/search-a-2d-matrix.py
rename to problems/python/search-a-2d-matrix.py
diff --git a/problems/search-in-a-binary-search-tree.py b/problems/python/search-in-a-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/search-in-a-binary-search-tree.py
rename to problems/python/search-in-a-binary-search-tree.py
diff --git a/problems/search-in-rotated-sorted-array-ii.py b/problems/python/search-in-rotated-sorted-array-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/search-in-rotated-sorted-array-ii.py
rename to problems/python/search-in-rotated-sorted-array-ii.py
diff --git a/problems/search-in-rotated-sorted-array.py b/problems/python/search-in-rotated-sorted-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/search-in-rotated-sorted-array.py
rename to problems/python/search-in-rotated-sorted-array.py
diff --git a/problems/search-insert-position.py b/problems/python/search-insert-position.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/search-insert-position.py
rename to problems/python/search-insert-position.py
diff --git a/problems/search-suggestions-system.py b/problems/python/search-suggestions-system.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/search-suggestions-system.py
rename to problems/python/search-suggestions-system.py
diff --git a/problems/second-highest-salary.sql b/problems/python/second-highest-salary.sql
old mode 100644
new mode 100755
similarity index 100%
rename from problems/second-highest-salary.sql
rename to problems/python/second-highest-salary.sql
diff --git a/problems/sell-diminishing-valued-colored-balls.py b/problems/python/sell-diminishing-valued-colored-balls.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sell-diminishing-valued-colored-balls.py
rename to problems/python/sell-diminishing-valued-colored-balls.py
diff --git a/problems/serialize-and-deserialize-binary-tree.py b/problems/python/serialize-and-deserialize-binary-tree.py
old mode 100644
new mode 100755
similarity index 64%
rename from problems/serialize-and-deserialize-binary-tree.py
rename to problems/python/serialize-and-deserialize-binary-tree.py
index 9519b74..e48f9d2
--- a/problems/serialize-and-deserialize-binary-tree.py
+++ b/problems/python/serialize-and-deserialize-binary-tree.py
@@ -39,4 +39,25 @@ def deserialize(self, data):
                 q.append(node.right)
             i += 1
             
-        return root
\ No newline at end of file
+        return root
+
+
+
+#Preorder traversal
+class Codec:
+    def serialize(self, root):
+        if not root: return '#,'
+        return str(root.val) + ',' + self.serialize(root.left) + self.serialize(root.right)
+    
+    def deserialize(self, data):
+        data = data.split(',')[::-1]
+        return self.getNode(data)
+    
+    def getNode(self, data):
+        if not data: return None
+        val = data.pop()
+        if val=='#': return None
+        node = TreeNode(val)
+        node.left = self.getNode(data)
+        node.right = self.getNode(data)
+        return node
\ No newline at end of file
diff --git a/problems/serialize-and-deserialize-bst.py b/problems/python/serialize-and-deserialize-bst.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/serialize-and-deserialize-bst.py
rename to problems/python/serialize-and-deserialize-bst.py
diff --git a/problems/set-matrix-zeroes.py b/problems/python/set-matrix-zeroes.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/set-matrix-zeroes.py
rename to problems/python/set-matrix-zeroes.py
diff --git a/problems/shortest-bridge.py b/problems/python/shortest-bridge.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/shortest-bridge.py
rename to problems/python/shortest-bridge.py
diff --git a/problems/shortest-common-supersequence.py b/problems/python/shortest-common-supersequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/shortest-common-supersequence.py
rename to problems/python/shortest-common-supersequence.py
diff --git a/problems/python/shortest-distance-from-all-buildings.py b/problems/python/shortest-distance-from-all-buildings.py
new file mode 100755
index 0000000..490e6a2
--- /dev/null
+++ b/problems/python/shortest-distance-from-all-buildings.py
@@ -0,0 +1,39 @@
+class Solution(object):
+    def shortestDistance(self, grid):
+        def bfs(i0, j0):
+            q = collections.deque([(i0, j0, 0)])
+            visited = set()
+            while q:
+                i, j, dis = q.popleft()
+                if (i, j) in visited: continue
+                visited.add((i, j))
+                
+                reach[i][j] += 1
+                distances[i][j] += dis
+                
+                for iNext, jNext in ((i+1, j), (i-1, j), (i, j+1), (i, j-1)):
+                    if not (0<=iNext<len(grid) and 0<=jNext<len(grid[0])): continue
+                    if grid[iNext][jNext]==2 or grid[iNext][jNext]==1: continue
+                    if (iNext, jNext) in visited: continue
+                    q.append((iNext, jNext, dis+1))
+        
+        M = len(grid)
+        N = len(grid[0])
+        ans = float('inf')
+        reach = [[0]*N for _ in xrange(M)] #number of buildings can reach (i, j)
+        distances = [[0]*N for _ in xrange(M)] #aggregate distance btwn 0 and buildings.
+        buildingCount = 0
+        
+        for i in xrange(M):
+            for j in xrange(N):
+                if grid[i][j]==1:
+                    buildingCount += 1
+                    bfs(i, j)
+        
+        
+        for i in xrange(M):
+            for j in xrange(N):
+                if grid[i][j]==0 and reach[i][j]==buildingCount:
+                    ans = min(ans, distances[i][j])
+        
+        return ans if ans!=float('inf') else -1
\ No newline at end of file
diff --git a/problems/python/shortest-path-in-a-grid-with-obstacles-elimination.py b/problems/python/shortest-path-in-a-grid-with-obstacles-elimination.py
new file mode 100755
index 0000000..9c94c8a
--- /dev/null
+++ b/problems/python/shortest-path-in-a-grid-with-obstacles-elimination.py
@@ -0,0 +1,25 @@
+class Solution(object):
+    def shortestPath(self, grid, K):
+        N = len(grid)
+        M = len(grid[0])
+        
+        q = collections.deque([(0, 0, 0, K)])
+        qNext = collections.deque()
+        visited = set()
+        
+        while q:
+            step, i0, j0, k0 = q.popleft()
+            if (i0, j0, k0) in visited: continue
+            visited.add((i0, j0, k0))
+            
+            if i0==N-1 and j0==M-1: return step
+            
+            for i, j in [(i0+1, j0), (i0-1, j0), (i0, j0+1), (i0, j0-1)]:
+                if i>=N or i<0 or j>=M or j<0: continue
+                if grid[i][j]==1 and k0>0:
+                    qNext.append((step+1, i, j, k0-1))
+                elif grid[i][j]==0:
+                    qNext.append((step+1, i, j, k0))
+            if not q: q = qNext
+        
+        return -1
\ No newline at end of file
diff --git a/problems/python/shortest-path-in-binary-matrix.py b/problems/python/shortest-path-in-binary-matrix.py
new file mode 100755
index 0000000..6811948
--- /dev/null
+++ b/problems/python/shortest-path-in-binary-matrix.py
@@ -0,0 +1,17 @@
+class Solution(object):
+    def shortestPathBinaryMatrix(self, grid):
+        q = collections.deque([(0, 0, 1)])
+        seen = 2
+        
+        while q:
+            i, j, step = q.popleft()
+            if not (0<=i<len(grid) and 0<=j<len(grid[0])): continue
+            if grid[i][j]==1 or grid[i][j]==seen: continue
+            grid[i][j] = seen
+            
+            if i==len(grid)-1 and j==len(grid[0])-1: return step
+            
+            for k, l in ((i+1, j), (i-1, j), (i, j+1), (i, j-1), (i-1, j-1), (i+1, j-1), (i+1, j+1), (i-1, j+1)):
+                q.append((k, l, step+1))
+        
+        return -1
\ No newline at end of file
diff --git a/problems/shortest-path-to-get-food.py b/problems/python/shortest-path-to-get-food.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/shortest-path-to-get-food.py
rename to problems/python/shortest-path-to-get-food.py
diff --git a/problems/shuffle-an-array.py b/problems/python/shuffle-an-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/shuffle-an-array.py
rename to problems/python/shuffle-an-array.py
diff --git a/problems/python/simplify-path.py b/problems/python/simplify-path.py
new file mode 100755
index 0000000..46589cf
--- /dev/null
+++ b/problems/python/simplify-path.py
@@ -0,0 +1,18 @@
+class Solution(object):
+    def simplifyPath(self, path):
+        ans = ''
+        skip = 0
+        
+        for directory in reversed(path.split('/')):
+            if directory=='' or directory=='.':
+                continue
+            elif directory=='..':
+                skip += 1
+            else:
+                if skip>0:
+                    skip -= 1
+                    continue
+                else:
+                    ans = '/'+directory+ans
+        
+        return ans if ans!='' else '/'
\ No newline at end of file
diff --git a/problems/python/single-threaded-cpu.py b/problems/python/single-threaded-cpu.py
new file mode 100755
index 0000000..fe30e21
--- /dev/null
+++ b/problems/python/single-threaded-cpu.py
@@ -0,0 +1,22 @@
+class Solution(object):
+    def getOrder(self, tasks):
+        ans = []
+        tasks = sorted([(task[0], task[1], i) for i, task in enumerate(tasks)], reverse=True)
+        pq = [] #tasks available
+        now = 0
+        
+        
+        while tasks or pq:
+            #check if the task is availiable, if yes, add to pq
+            while tasks and tasks[-1][0]<=now:
+                startTime, processTime, i = tasks.pop()
+                heapq.heappush(pq, (processTime, i))
+                        
+            if pq:
+                processTime, i = heapq.heappop(pq)
+                ans.append(i)
+                now += processTime
+            else:
+                now = tasks[-1][0]
+                    
+        return ans
\ No newline at end of file
diff --git a/problems/sliding-window-maximum.py b/problems/python/sliding-window-maximum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sliding-window-maximum.py
rename to problems/python/sliding-window-maximum.py
diff --git a/problems/python/snapshot-array.py b/problems/python/snapshot-array.py
new file mode 100755
index 0000000..baa9f15
--- /dev/null
+++ b/problems/python/snapshot-array.py
@@ -0,0 +1,19 @@
+class SnapshotArray(object):
+
+    def __init__(self, length):
+        self.data = [[(-1, 0)] for _ in xrange(length)]
+        self.snapId = 0
+        
+        
+    def set(self, index, val):
+        self.data[index].append((self.snapId, val))
+        
+        
+    def snap(self):
+        self.snapId += 1
+        return self.snapId - 1
+            
+        
+    def get(self, index, snapId):
+        j = bisect.bisect_right(self.data[index], (snapId, float('inf'))) - 1
+        return self.data[index][j][1]
\ No newline at end of file
diff --git a/problems/sort-colors.py b/problems/python/sort-colors.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sort-colors.py
rename to problems/python/sort-colors.py
diff --git a/problems/sort-list.py b/problems/python/sort-list.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sort-list.py
rename to problems/python/sort-list.py
diff --git a/problems/spiral-matrix.py b/problems/python/spiral-matrix.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/spiral-matrix.py
rename to problems/python/spiral-matrix.py
diff --git a/problems/split-array-into-fibonacci-sequence.py b/problems/python/split-array-into-fibonacci-sequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/split-array-into-fibonacci-sequence.py
rename to problems/python/split-array-into-fibonacci-sequence.py
diff --git a/problems/split-array-largest-sum.py b/problems/python/split-array-largest-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/split-array-largest-sum.py
rename to problems/python/split-array-largest-sum.py
diff --git a/problems/sqrtx.py b/problems/python/sqrtx.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sqrtx.py
rename to problems/python/sqrtx.py
diff --git a/problems/squares-of-a-sorted-array.py b/problems/python/squares-of-a-sorted-array.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/squares-of-a-sorted-array.py
rename to problems/python/squares-of-a-sorted-array.py
diff --git a/problems/python/step-by-step-directions-from-a-binary-tree-node-to-another.py b/problems/python/step-by-step-directions-from-a-binary-tree-node-to-another.py
new file mode 100755
index 0000000..a9177d4
--- /dev/null
+++ b/problems/python/step-by-step-directions-from-a-binary-tree-node-to-another.py
@@ -0,0 +1,36 @@
+class Solution(object):
+    def __init__(self):
+        self.lca = None
+        
+    def getDirections(self, root, startValue, destValue):
+        def findPath(node, path, val):
+            if not node: return False
+            if node.val==val: return True
+            
+            path.append('L')
+            if findPath(node.left, path, val): return True
+            path.pop()
+            
+            path.append('R')
+            if findPath(node.right, path, val): return True
+            path.pop()
+            
+            return False
+            
+        def findCount(node, startValue, destValue):
+            if not node: return 0
+            count = 0
+            if node.val==startValue or node.val==destValue: count += 1
+            count += findCount(node.left, startValue, destValue)
+            count += findCount(node.right, startValue, destValue)
+            if count>=2 and not self.lca: self.lca = node
+            return count
+        
+        findCount(root, startValue, destValue)
+        
+        path1 = []
+        findPath(self.lca, path1, startValue)
+        path2 = []
+        findPath(self.lca, path2, destValue)
+        
+        return 'U'*len(path1) + ''.join(path2)
\ No newline at end of file
diff --git a/problems/python/stock-price-fluctuation.py b/problems/python/stock-price-fluctuation.py
new file mode 100755
index 0000000..e75beb2
--- /dev/null
+++ b/problems/python/stock-price-fluctuation.py
@@ -0,0 +1,37 @@
+"""
+[0] priceToTime tracks price to timestamps, but since for each timestamp the price might be overridden, the price in priceToTime might not be exsit. So we need to check if the price still have any valid timestamps.
+[1] SortedDict is a BST like structure.
+
+Time: O(LogN)
+Space: O(N)
+"""
+from sortedcontainers import SortedDict #[1]
+
+class StockPrice(object):
+
+    def __init__(self):
+        self.timeToPrice = SortedDict() #time will be sorted
+        self.priceToTime = SortedDict() #the price will be sorted
+        
+
+    def update(self, timestamp, price):
+        if timestamp in self.timeToPrice:
+            prevPrice = self.timeToPrice[timestamp]
+            self.priceToTime[prevPrice].remove(timestamp)
+            if len(self.priceToTime[prevPrice])==0: self.priceToTime.pop(prevPrice) #[0]
+        
+        if price not in self.priceToTime: self.priceToTime[price] = set() #initialized
+        self.priceToTime[price].add(timestamp)
+        self.timeToPrice[timestamp] = price
+            
+
+    def current(self):
+        return self.timeToPrice.peekitem(-1)[1]
+        
+
+    def maximum(self):
+        return self.priceToTime.peekitem(-1)[0]
+        
+
+    def minimum(self):
+        return self.priceToTime.peekitem(0)[0]
\ No newline at end of file
diff --git a/problems/stone-game-ii.py b/problems/python/stone-game-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/stone-game-ii.py
rename to problems/python/stone-game-ii.py
diff --git a/problems/python/student-attendance-record-ii.py b/problems/python/student-attendance-record-ii.py
new file mode 100755
index 0000000..efc2923
--- /dev/null
+++ b/problems/python/student-attendance-record-ii.py
@@ -0,0 +1,27 @@
+class Solution(object):
+    def checkRecord(self, n):
+        """
+        dp[l] := number of eligible combination of a length l record without A
+        """
+        
+        ans = 0
+        M = 1000000007
+        
+        dp = [0]*max(n+1, 4)
+        dp[0] = 1
+        dp[1] = 2
+        dp[2] = 4
+        dp[3] = 7
+        
+        for i in xrange(4, n+1):
+            dp[i] += dp[i-1]%M #ends at P
+            dp[i] += (dp[i-1]%M - dp[i-4]%M) #ends at L. All posiblity but the end cannot be PLL
+        
+        ans += dp[n]
+        
+        for i in xrange(n):
+            ans += dp[i] * dp[n-i-1]
+            ans %= M
+            
+        return ans
+        
\ No newline at end of file
diff --git a/problems/subarray-sum-equals-k.py b/problems/python/subarray-sum-equals-k.py
old mode 100644
new mode 100755
similarity index 97%
rename from problems/subarray-sum-equals-k.py
rename to problems/python/subarray-sum-equals-k.py
index 3220cff..0158b04
--- a/problems/subarray-sum-equals-k.py
+++ b/problems/python/subarray-sum-equals-k.py
@@ -39,7 +39,6 @@ def subarraySum(self, nums, k):
 """
 class Solution(object):
     def subarraySum(self, nums, k):
-        N = len(nums)
         ans = 0
         
         prefixSumCount = collections.Counter()
@@ -52,4 +51,4 @@ def subarraySum(self, nums, k):
             ans += prefixSumCount[I] #there are "prefixSumCount[I]" combinations of J-I that equals to k.
             prefixSumCount[J] += 1
             
-        return ans
\ No newline at end of file
+        return ans
diff --git a/problems/python/subarrays-with-k-different-integers.py b/problems/python/subarrays-with-k-different-integers.py
new file mode 100755
index 0000000..5753427
--- /dev/null
+++ b/problems/python/subarrays-with-k-different-integers.py
@@ -0,0 +1,25 @@
+class Solution(object):
+    def subarraysWithKDistinct(self, nums, K):
+        #number of subarray of nums which have at most k different numbers.
+        def atMost(k):
+            i = 0
+            ans = 0
+            counter = collections.Counter()
+            uniqueCount = 0
+            
+            for j in xrange(len(nums)):
+                counter[nums[j]] += 1
+                if counter[nums[j]]==1: uniqueCount += 1
+                
+                while uniqueCount>k:
+                    counter[nums[i]] -= 1
+                    if counter[nums[i]]==0: uniqueCount -= 1
+                    i += 1
+                
+                # the logest subarray that ends at j is nums[i:j+1]
+                # nums[i:j+1] can produce j-i+1 subarrays that at most has k different number.
+                ans += j-i+1
+            
+            return ans
+        
+        return atMost(K)-atMost(K-1)
\ No newline at end of file
diff --git a/problems/subdomain-visit-count.py b/problems/python/subdomain-visit-count.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/subdomain-visit-count.py
rename to problems/python/subdomain-visit-count.py
diff --git a/problems/subsets-ii.py b/problems/python/subsets-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/subsets-ii.py
rename to problems/python/subsets-ii.py
diff --git a/problems/subsets.py b/problems/python/subsets.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/subsets.py
rename to problems/python/subsets.py
diff --git a/problems/substring-with-concatenation-of-all-words.py b/problems/python/substring-with-concatenation-of-all-words.py
old mode 100644
new mode 100755
similarity index 99%
rename from problems/substring-with-concatenation-of-all-words.py
rename to problems/python/substring-with-concatenation-of-all-words.py
index b11bb81..e3e6420
--- a/problems/substring-with-concatenation-of-all-words.py
+++ b/problems/python/substring-with-concatenation-of-all-words.py
@@ -47,4 +47,6 @@ def test(self, s, wl, countExpected):
             
         return True
         
-        
\ No newline at end of file
+
+
+
diff --git a/problems/subtree-of-another-tree.py b/problems/python/subtree-of-another-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/subtree-of-another-tree.py
rename to problems/python/subtree-of-another-tree.py
diff --git a/problems/sum-of-subarray-minimums.py b/problems/python/sum-of-subarray-minimums.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sum-of-subarray-minimums.py
rename to problems/python/sum-of-subarray-minimums.py
diff --git a/problems/sum-root-to-leaf-numbers.py b/problems/python/sum-root-to-leaf-numbers.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/sum-root-to-leaf-numbers.py
rename to problems/python/sum-root-to-leaf-numbers.py
diff --git a/problems/summary-ranges.py b/problems/python/summary-ranges.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/summary-ranges.py
rename to problems/python/summary-ranges.py
diff --git a/problems/super-ugly-number.py b/problems/python/super-ugly-number.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/super-ugly-number.py
rename to problems/python/super-ugly-number.py
diff --git a/problems/python/swap-adjacent-in-lr-string.py b/problems/python/swap-adjacent-in-lr-string.py
new file mode 100755
index 0000000..a8acf65
--- /dev/null
+++ b/problems/python/swap-adjacent-in-lr-string.py
@@ -0,0 +1,18 @@
+class Solution(object):
+    def canTransform(self, start, end):
+        if len(start)!=len(end): return False
+        if start.replace('X', '')!=end.replace('X', ''): return False
+        
+        startLIndex = [i for i, c in enumerate(start) if c=='L']
+        endLIndex = [i for i, c in enumerate(end) if c=='L']
+        for i in xrange(len(startLIndex)):
+            if startLIndex[i]<endLIndex[i]:
+                return False
+        
+        startRIndex = [i for i, c in enumerate(start) if c=='R']
+        endRIndex = [i for i, c in enumerate(end) if c=='R']
+        for i in xrange(len(startRIndex)):
+            if startRIndex[i]>endRIndex[i]:
+                return False
+        
+        return True
\ No newline at end of file
diff --git a/problems/swap-nodes-in-pairs.py b/problems/python/swap-nodes-in-pairs.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/swap-nodes-in-pairs.py
rename to problems/python/swap-nodes-in-pairs.py
diff --git a/problems/swim-in-rising-water.py b/problems/python/swim-in-rising-water.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/swim-in-rising-water.py
rename to problems/python/swim-in-rising-water.py
diff --git a/problems/symmetric-tree.py b/problems/python/symmetric-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/symmetric-tree.py
rename to problems/python/symmetric-tree.py
diff --git a/problems/tallest-billboard.py b/problems/python/tallest-billboard.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/tallest-billboard.py
rename to problems/python/tallest-billboard.py
diff --git a/problems/target-sum.py b/problems/python/target-sum.py
old mode 100644
new mode 100755
similarity index 72%
rename from problems/target-sum.py
rename to problems/python/target-sum.py
index 75d9abe..c216522
--- a/problems/target-sum.py
+++ b/problems/python/target-sum.py
@@ -55,4 +55,23 @@ def findTargetSumWays(self, nums, target):
             for t in xrange(minTarget, maxTarget+1):
                 dp[i][t] = (dp[i-1][t-nums[i-1]] if t-nums[i-1]>=minTarget else 0) + (dp[i-1][t+nums[i-1]] if t+nums[i-1]<=maxTarget else 0)
         
-        return dp[N][target]
\ No newline at end of file
+        return dp[N][target]
+
+
+"""
+dp[i][s] := considerting nums[:i] how many expressions can sum up to s.
+dp[0][0] = 1
+"""
+class Solution(object):
+    def findTargetSumWays(self, nums, target):
+        S = sum(nums)
+        if not -S<=target<=S: return 0
+        
+        dp = [{s:0 for s in xrange(-S, S+1)} for _ in xrange(len(nums)+1)]
+        dp[0][0] = 1
+        
+        for i in xrange(1, len(nums)+1):
+            for s in xrange(-S, S+1):
+                dp[i][s] = (dp[i-1][s+nums[i-1]] if s+nums[i-1]<=S else 0) + (dp[i-1][s-nums[i-1]] if s-nums[i-1]>=-S else 0)
+        
+        return dp[-1][target]
\ No newline at end of file
diff --git a/problems/task-scheduler.py b/problems/python/task-scheduler.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/task-scheduler.py
rename to problems/python/task-scheduler.py
diff --git a/problems/python/text-justification.py b/problems/python/text-justification.py
new file mode 100755
index 0000000..98eb0be
--- /dev/null
+++ b/problems/python/text-justification.py
@@ -0,0 +1,68 @@
+"""
+The description of this problem is unclear.
+"Extra spaces between words should be distributed as evenly as possible." Should be "Extra spaces between words IN EACH LINE should be distributed as evenly as possible."
+So the problem in short is:
+1. Every line should have as many as word as possible. But they should be separate by space or spaces.
+2. Mid align all the line. Except the last line and the lines with one word.
+3. For mid align, distribute space evenly, if there are extra spaces, it should be located left. For example 5 spaces distribute to 2 places, it should be 3 2, not 2 3. 7 spaces distribute to 3 places, it should be 3 2 2, not 2 3 2, not 2 2 3.
+
+Time: O(N), N is the number of words.
+Space: O(W), W is maxWidth for keeping strings in currLine. Can further reduce to O(1) using only index.
+"""
+class Solution(object):
+    def fullJustify(self, words, maxWidth):
+        def canAddToCurrLine(word, currLineStringLength):
+            currWidth = len(currLine)-1 + currLineStringLength # space+currLineStringLength
+            return currWidth+len(word)+1<=maxWidth
+        
+        def leftAlign(currLine):
+            line = ''
+            
+            for word in currLine:
+                line += word + ' '
+            line = line[:-1] #remove last space
+            
+            line += ' '*(maxWidth-len(line))
+            return line
+        
+        def midAlign(currLine, currLineStringLength):
+            line = ''
+            
+            totalSpaceCount = maxWidth-currLineStringLength
+            extraSpaceCount = totalSpaceCount%(len(currLine)-1)
+            spaceCount = totalSpaceCount-extraSpaceCount
+            spaces = ' '*(totalSpaceCount/(len(currLine)-1))
+            
+            for word in currLine:
+                line += word
+                if spaceCount>0:
+                    line += spaces
+                    spaceCount -= len(spaces)
+                
+                if extraSpaceCount>0:
+                    line += ' '
+                    extraSpaceCount -= 1
+                    
+            return line
+            
+        currLineStringLength = 0
+        currLine = []
+        ans = []
+        
+        i = 0
+        while i<len(words):
+            if canAddToCurrLine(words[i], currLineStringLength):
+                currLine.append(words[i])
+                currLineStringLength += len(words[i])
+                i += 1
+            else:
+                if len(currLine)==1:
+                    ans.append(leftAlign(currLine)) #line with one word align left
+                else:
+                    ans.append(midAlign(currLine, currLineStringLength))
+                
+                currLine = []
+                currLineStringLength = 0
+                
+        ans.append(leftAlign(currLine)) #last line should always align left
+        return ans
\ No newline at end of file
diff --git a/problems/the-kth-factor-of-n.py b/problems/python/the-kth-factor-of-n.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/the-kth-factor-of-n.py
rename to problems/python/the-kth-factor-of-n.py
diff --git a/problems/python/the-maze-ii.py b/problems/python/the-maze-ii.py
new file mode 100755
index 0000000..6ab19bc
--- /dev/null
+++ b/problems/python/the-maze-ii.py
@@ -0,0 +1,45 @@
+class Solution(object):
+    def shortestDistance(self, maze, start, destination):
+        N, M = len(maze), len(maze[0])
+        i0, j0 = start
+        pq = [(0, i0, j0, 'stop')]
+        visited = set()
+        
+        
+        while pq:
+            dis, i, j, direction = heapq.heappop(pq)
+            if (i, j, direction) in visited: continue
+            visited.add((i, j, direction))
+            
+            if i<0 or i>=N or j<0 or j>=M: continue
+            if maze[i][j]==1: continue
+            
+            if i==destination[0] and j==destination[1] and direction=='stop': return dis
+                
+            if direction=='stop':
+                heapq.heappush(pq, (dis+1, i-1, j, 'left'))
+                heapq.heappush(pq, (dis+1, i+1, j, 'right'))
+                heapq.heappush(pq, (dis+1, i, j-1, 'down'))
+                heapq.heappush(pq, (dis+1, i, j+1, 'up'))
+            elif direction=='left':
+                if i-1<0 or i-1>=N or j<0 or j>=M or maze[i-1][j]==1:
+                    heapq.heappush(pq, (dis, i, j, 'stop'))
+                else:
+                    heapq.heappush(pq, (dis+1, i-1, j, direction))
+            elif direction=='right':
+                if i+1<0 or i+1>=N or j<0 or j>=M or maze[i+1][j]==1:
+                    heapq.heappush(pq, (dis, i, j, 'stop'))
+                else:
+                    heapq.heappush(pq, (dis+1, i+1, j, direction))
+            elif direction=='down':
+                if i<0 or i>=N or j-1<0 or j-1>=M or maze[i][j-1]==1:
+                    heapq.heappush(pq, (dis, i, j, 'stop'))
+                else:
+                    heapq.heappush(pq, (dis+1, i, j-1, direction))
+            elif direction=='up':
+                if i<0 or i>=N or j+1<0 or j+1>=M or maze[i][j+1]==1:
+                    heapq.heappush(pq, (dis, i, j, 'stop'))
+                else:
+                    heapq.heappush(pq, (dis+1, i, j+1, direction))
+                    
+        return -1
\ No newline at end of file
diff --git a/problems/time-based-key-value-store.py b/problems/python/time-based-key-value-store.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/time-based-key-value-store.py
rename to problems/python/time-based-key-value-store.py
diff --git a/problems/to-lower-case.py b/problems/python/to-lower-case.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/to-lower-case.py
rename to problems/python/to-lower-case.py
diff --git a/problems/python/toeplitz-matrix.py b/problems/python/toeplitz-matrix.py
new file mode 100755
index 0000000..431ce27
--- /dev/null
+++ b/problems/python/toeplitz-matrix.py
@@ -0,0 +1,22 @@
+class Solution(object):
+    def isToeplitzMatrix(self, matrix):
+        def check(i0, j0, matrix):
+            i = i0
+            j = j0
+            
+            while 0<=i<len(matrix) and 0<=j<len(matrix[0]):
+                if matrix[i][j]!=matrix[i0][j0]: return False
+                i += 1
+                j += 1
+            return True
+    
+        M = len(matrix)
+        N = len(matrix[0])
+        
+        for i in xrange(M-1, 0, -1):
+            if not check(i, 0, matrix): return False
+        
+        for j in xrange(N):
+            if not check(0, j, matrix): return False
+        
+        return True
\ No newline at end of file
diff --git a/problems/top-k-frequent-elements.py b/problems/python/top-k-frequent-elements.py
old mode 100644
new mode 100755
similarity index 66%
rename from problems/top-k-frequent-elements.py
rename to problems/python/top-k-frequent-elements.py
index fbb68a6..e1dea8c
--- a/problems/top-k-frequent-elements.py
+++ b/problems/python/top-k-frequent-elements.py
@@ -88,4 +88,62 @@ def topKFrequent(self, nums, k):
             ans.append(num)
             k -= 1
         
+        return ans
+
+"""
+Bucket sort for counts.
+Space/Time: O(N)
+"""
+class Solution(object):
+    def topKFrequent(self, nums, k):
+        counter = collections.Counter(nums)
+        count2Nums = [[] for _ in xrange(len(nums)+1)] #count2Nums[i] := elements with count i
+        
+        for num in counter:
+            count2Nums[counter[num]].append(num)
+        
+        ans = []
+        for i in xrange(len(count2Nums)-1, -1, -1):
+            ans += count2Nums[i]
+            if len(ans)>=k: break
+        
+        return ans
+
+
+# Quick Select
+class Solution(object):
+    def topKFrequent(self, nums, K):
+        def quickselect(A, s, e, K):
+            i = s
+            t = s
+            j = e
+            
+            pivot = A[(s+e)/2][0]
+            while t<=j:
+                if A[t][0]<pivot:
+                    A[t], A[i] = A[i], A[t]
+                    i += 1
+                    t += 1
+                elif A[t][0]==pivot:
+                    t += 1
+                else:
+                    A[t], A[j] = A[j], A[t]
+                    j -=1
+            
+            if e-j>=K:
+                return quickselect(A, j+1, e, K)
+            elif e-(i-1)>=K:
+                return pivot
+            else:
+                return quickselect(A, s, i-1, K-(e-i+1))
+                
+        ans = []
+        counter = collections.Counter(nums)
+        freqs = [(counter[num], num) for num in counter]
+        
+        KthLargestFreq = quickselect(freqs, 0, len(freqs)-1, K)
+        
+        for freq, num in freqs:
+            if freq>=KthLargestFreq: ans.append(num)
+                
         return ans
\ No newline at end of file
diff --git a/problems/python/trapping-rain-water-ii.py b/problems/python/trapping-rain-water-ii.py
new file mode 100755
index 0000000..dd14bc7
--- /dev/null
+++ b/problems/python/trapping-rain-water-ii.py
@@ -0,0 +1,28 @@
+class Solution(object):
+    def trapRainWater(self, heightMap):
+        pq = []
+        N = len(heightMap)
+        M = len(heightMap[0])
+        visited = set()
+        ans = 0
+        curr = float('-inf')
+        
+        for i in xrange(N):
+            for j in xrange(M):
+                if i==0 or i==N-1 or j==0 or j==M-1:
+                    heapq.heappush(pq, (heightMap[i][j], i, j))
+                
+        while pq:
+            h, i, j = heapq.heappop(pq)
+            if (i, j) in visited: continue
+            visited.add((i, j))
+            
+            if h>curr: curr = h
+            ans += (curr-h)
+            
+            for iNext, jNext in [(i+1, j), (i-1, j), (i, j+1), (i, j-1)]:
+                if iNext<0 or iNext>=N or jNext<0 or jNext>=M: continue
+                if (iNext, jNext) in visited: continue
+                heapq.heappush(pq, (heightMap[iNext][jNext], iNext, jNext))
+        
+        return ans
\ No newline at end of file
diff --git a/problems/trapping-rain-water.py b/problems/python/trapping-rain-water.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/trapping-rain-water.py
rename to problems/python/trapping-rain-water.py
diff --git a/problems/trim-a-binary-search-tree.py b/problems/python/trim-a-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/trim-a-binary-search-tree.py
rename to problems/python/trim-a-binary-search-tree.py
diff --git a/problems/python/two-out-of-three.py b/problems/python/two-out-of-three.py
new file mode 100755
index 0000000..8518903
--- /dev/null
+++ b/problems/python/two-out-of-three.py
@@ -0,0 +1,17 @@
+class Solution(object):
+    def twoOutOfThree(self, nums1, nums2, nums3):
+        ans = []
+        
+        counter = collections.Counter()
+        
+        for num in list(set(nums1)):
+            counter[num] += 1
+        for num in list(set(nums2)):
+            counter[num] += 1
+        for num in list(set(nums3)):
+            counter[num] += 1
+        
+        for num in counter:
+            if counter[num]>=2: ans.append(num)
+        
+        return ans
\ No newline at end of file
diff --git a/problems/two-sum-ii-input-array-is-sorted.py b/problems/python/two-sum-ii-input-array-is-sorted.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/two-sum-ii-input-array-is-sorted.py
rename to problems/python/two-sum-ii-input-array-is-sorted.py
diff --git a/problems/two-sum.py b/problems/python/two-sum.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/two-sum.py
rename to problems/python/two-sum.py
diff --git a/problems/ugly-number-ii.py b/problems/python/ugly-number-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/ugly-number-ii.py
rename to problems/python/ugly-number-ii.py
diff --git a/problems/ugly-number.py b/problems/python/ugly-number.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/ugly-number.py
rename to problems/python/ugly-number.py
diff --git a/problems/umber-of-islands-ii.py b/problems/python/umber-of-islands-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/umber-of-islands-ii.py
rename to problems/python/umber-of-islands-ii.py
diff --git a/problems/unique-binary-search-trees-ii,py b/problems/python/unique-binary-search-trees-ii,py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/unique-binary-search-trees-ii,py
rename to problems/python/unique-binary-search-trees-ii,py
diff --git a/problems/unique-binary-search-trees.py b/problems/python/unique-binary-search-trees.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/unique-binary-search-trees.py
rename to problems/python/unique-binary-search-trees.py
diff --git a/problems/unique-email-addres.py b/problems/python/unique-email-addres.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/unique-email-addres.py
rename to problems/python/unique-email-addres.py
diff --git a/problems/unique-paths.py b/problems/python/unique-paths.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/unique-paths.py
rename to problems/python/unique-paths.py
diff --git a/problems/univalued-binary-tree.py b/problems/python/univalued-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/univalued-binary-tree.py
rename to problems/python/univalued-binary-tree.py
diff --git a/problems/valid-anagram.py b/problems/python/valid-anagram.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/valid-anagram.py
rename to problems/python/valid-anagram.py
diff --git a/problems/python/valid-number.py b/problems/python/valid-number.py
new file mode 100755
index 0000000..9d0793a
--- /dev/null
+++ b/problems/python/valid-number.py
@@ -0,0 +1,44 @@
+class Solution(object):
+    def isNumber(self, s):
+        def isOK(s, maxDots):
+            if not s: return False
+            
+            #check and remove +/-
+            for i, c in enumerate(s):
+                if (c=='+' or c=='-') and i!=0:
+                    return False
+            if s[0]=='+' or s[0]=='-': s = s[1:]
+            
+            #check dot and if there is digit
+            dotCount = 0
+            dotPos = 0
+            digitCount = 0
+            for i, c in enumerate(s):
+                if c=='.':
+                    dotCount += 1
+                    dotPos = i
+                elif c.isdigit():
+                    digitCount += 1
+            
+            if dotCount>maxDots: return False
+            if digitCount==0: return False
+            
+            return True
+        
+        #get e's position. Also check if all char is in validChar
+        validChar = set(['1','2', '3', '4', '5', '6', '7', '8', '9', '0', 'e', 'E', '.', '+', '-'])
+        eCount = 0
+        ePos = 0
+        for i, c in enumerate(s):
+            if c not in validChar:
+                return False
+            if c=='e' or c=='E':
+                eCount += 1
+                ePos = i
+            
+        if eCount>1:
+            return False
+        elif eCount==1:
+            return isOK(s[:ePos], 1) and isOK(s[ePos+1:], 0)
+        else:
+            return isOK(s, 1)
diff --git a/problems/python/valid-palindrome-ii.py b/problems/python/valid-palindrome-ii.py
new file mode 100755
index 0000000..84bce52
--- /dev/null
+++ b/problems/python/valid-palindrome-ii.py
@@ -0,0 +1,29 @@
+"""
+When seeing the first different chars in the string.
+Check if anyone of the string is palindrome if we remove one of them.
+
+Time: O(N)
+Space: O(1)
+"""
+class Solution(object):
+    def validPalindrome(self, s):
+        def isPalindrome(s):
+            i = 0
+            j = len(s)-1
+            while i<j:
+                if s[i]!=s[j]: return False
+                i += 1
+                j -= 1
+            return True
+        
+        i = 0
+        j = len(s)-1
+        
+        while i<j:
+            if s[i]==s[j]:
+                i += 1
+                j -= 1
+            else:
+                return isPalindrome(s[i:j]) or isPalindrome(s[i+1:j+1])
+            
+        return True
\ No newline at end of file
diff --git a/problems/valid-palindrome.py b/problems/python/valid-palindrome.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/valid-palindrome.py
rename to problems/python/valid-palindrome.py
diff --git a/problems/valid-parentheses.py b/problems/python/valid-parentheses.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/valid-parentheses.py
rename to problems/python/valid-parentheses.py
diff --git a/problems/python/valid-word-abbreviation.py b/problems/python/valid-word-abbreviation.py
new file mode 100755
index 0000000..630f1b2
--- /dev/null
+++ b/problems/python/valid-word-abbreviation.py
@@ -0,0 +1,30 @@
+class Solution(object):
+    def validWordAbbreviation(self, word, abbr):
+        #process abbr
+        splitedAbbr = []
+        for c in abbr:
+            if c.isdigit() and splitedAbbr and splitedAbbr[-1][0].isdigit():
+                splitedAbbr[-1] += c
+            elif c.isdigit() and c=='0':
+                return False #leading 0
+            else:
+                splitedAbbr.append(c)
+        for i, c in enumerate(splitedAbbr):
+            if c[0].isdigit(): splitedAbbr[i] = int(c)
+                    
+        i = 0
+        j = 0
+        while i<len(word) and j<len(splitedAbbr):
+            if word[i]==splitedAbbr[j]:
+                i += 1
+                j += 1
+                continue
+            
+            if isinstance(splitedAbbr[j], basestring):
+                return False
+            else:
+                splitedAbbr[j] -= 1
+                if splitedAbbr[j]==0: j += 1
+                i += 1
+        
+        return i==len(word) and j==len(splitedAbbr)
\ No newline at end of file
diff --git a/problems/validate-binary-search-tree.py b/problems/python/validate-binary-search-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/validate-binary-search-tree.py
rename to problems/python/validate-binary-search-tree.py
diff --git a/problems/verify-preorder-serialization-of-a-binary-tree.py b/problems/python/verify-preorder-serialization-of-a-binary-tree.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/verify-preorder-serialization-of-a-binary-tree.py
rename to problems/python/verify-preorder-serialization-of-a-binary-tree.py
diff --git a/problems/python/vertical-order-traversal-of-a-binary-tree.py b/problems/python/vertical-order-traversal-of-a-binary-tree.py
new file mode 100755
index 0000000..bda15ad
--- /dev/null
+++ b/problems/python/vertical-order-traversal-of-a-binary-tree.py
@@ -0,0 +1,67 @@
+"""
+The description wants us to report from left to right, top to bottom.
+Thus, we keep add value into `temp` with `x_position` as the key and `(y_position, node.val)` as the value.
+And maintain `min_x`, `max_x` at the same time. So we know how to iterate the `temp`.
+"""
+from collections import defaultdict
+
+class Solution(object):
+    def verticalTraversal(self, root):
+        temp = defaultdict(list)
+        min_x = 0
+        max_x = 0
+
+        stack = []
+        stack.append((root, 0, 0))
+        while stack:
+            node, x, y = stack.pop()
+            temp[x].append((y, node.val)) #append the value of height, so we can sort by height later on
+            min_x = min(min_x, x)
+            max_x = max(max_x, x)
+            if node.left: stack.append((node.left, x-1, y+1))
+            if node.right: stack.append((node.right, x+1, y+1))
+        
+        opt = []
+        for i in range(min_x, max_x+1):
+            opt.append([v for y, v in sorted(temp[i])]) #the temp[i] will be sorted by y_position then sorted by node.val
+        
+        return opt
+
+
+"""
+Time: O(Nk) in average case.
+dfs takes O(N).
+Double for-loop take nearly O(N), in the double for-loop we need to spend some time on sorting.
+But the number of elements in the same column and row is very very small. Lets say, O(k)
+
+Space: O(N)
+"""
+class Solution(object):
+    def verticalTraversal(self, root):
+        def dfs(node, x, y):
+            if not node: return
+            self.maxX = max(self.maxX, x)
+            self.minX = min(self.minX, x)
+            self.minY = min(self.minY, y)
+            data[(x, y)].append(node.val)
+            dfs(node.left, x-1, y-1)
+            dfs(node.right, x+1, y-1)
+        
+        data = collections.defaultdict(list)
+        self.maxX = float('-inf')
+        self.minX = float('inf')
+        self.minY = float('inf')
+        
+        dfs(root, 0, 0)
+        
+        ans = []
+        for x in xrange(self.minX, self.maxX+1):
+            temp = []
+            for y in xrange(0, self.minY-1, -1):
+                if (x, y) not in data: continue
+                if len(data[(x, y)])>1:
+                    temp += sorted(data[(x, y)])
+                else:
+                    temp.append(data[(x, y)][0])
+            ans.append(temp)
+        return ans
\ No newline at end of file
diff --git a/problems/wiggle-subsequence.py b/problems/python/wiggle-subsequence.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/wiggle-subsequence.py
rename to problems/python/wiggle-subsequence.py
diff --git a/problems/word-break.py b/problems/python/word-break.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/word-break.py
rename to problems/python/word-break.py
diff --git a/problems/word-ladder-ii.py b/problems/python/word-ladder-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/word-ladder-ii.py
rename to problems/python/word-ladder-ii.py
diff --git a/problems/word-ladder.py b/problems/python/word-ladder.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/word-ladder.py
rename to problems/python/word-ladder.py
diff --git a/problems/word-search-ii.py b/problems/python/word-search-ii.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/word-search-ii.py
rename to problems/python/word-search-ii.py
diff --git a/problems/word-search.py b/problems/python/word-search.py
old mode 100644
new mode 100755
similarity index 100%
rename from problems/word-search.py
rename to problems/python/word-search.py
diff --git a/problems/python3/3sum-closest.py b/problems/python3/3sum-closest.py
new file mode 100755
index 0000000..472c80f
--- /dev/null
+++ b/problems/python3/3sum-closest.py
@@ -0,0 +1,24 @@
+"""
+Time: O(N^2)
+Space: O(1)
+"""
+class Solution:
+    def threeSumClosest(self, nums: List[int], target: int) -> int:
+        nums.sort()
+        
+        N = len(nums)
+        ans = float('inf')
+        
+        for i in range(N):
+            j = i+1
+            k = N-1
+            
+            while j<k:
+                total = nums[i]+nums[j]+nums[k]
+                if total==target: return total
+                if abs(target-ans)>abs(target-total): ans = total
+                if total>target:
+                    k -= 1
+                elif total<target:
+                    j += 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/3sum-smaller.py b/problems/python3/3sum-smaller.py
new file mode 100755
index 0000000..3291586
--- /dev/null
+++ b/problems/python3/3sum-smaller.py
@@ -0,0 +1,18 @@
+class Solution:
+    def threeSumSmaller(self, nums: List[int], target: int) -> int:
+        N = len(nums)
+        ans = 0
+        
+        nums.sort()
+        
+        
+        for i in range(N):
+            j = i+1
+            k = N-1
+            while j<k:
+                if nums[i]+nums[j]+nums[k]<target:
+                    ans += k-j
+                    j += 1
+                else:
+                    k -= 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/3sum.py b/problems/python3/3sum.py
new file mode 100755
index 0000000..97f3388
--- /dev/null
+++ b/problems/python3/3sum.py
@@ -0,0 +1,92 @@
+"""
+Time: O(N^2)
+Space: O(1)
+
+First of all, we need to sort the list, so that it is easier to know where to move the pointers.
+(Keep in mind that sorting takes O(NLogN) of time, but since judging from the problem this solution might takes at least O(N^2), so it is ok to sort.)
+
+Now, we have 3 unique pointers at the list. i, j, k, where i<j<k.
+For a given i, set j at i+1 and k at the rightest.
+
+If the sum is too large, our only option is to move k left, so we can find smaller sum.
+If the sum is too small, our only option is to move j right, so we can find larger sum.
+If the sum is what we want, add it to the ans.
+
+[0] we need to skip the num that is the same as the last one to remove duplicate. This is actually the hardest part of the problem.
+[1] if nums[i] is larger than 0, we can stop, since nums[i] and nums[k] are surely larger than nums[i]
+"""
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        ans = []
+        nums.sort()
+        
+        for i in range(len(nums)):
+            if 0<i and nums[i]==nums[i-1]: continue #[0]
+            if nums[i]>0: break #[1]
+            
+            j, k = i+1, len(nums)-1
+            while j<k:
+                if nums[i]+nums[j]+nums[k]>0:
+                    k -= 1
+                elif nums[i]+nums[j]+nums[k]<0:
+                    j += 1
+                else:
+                    ans.append((nums[i], nums[j], nums[k]))
+                    while 0<k-1 and nums[k-1]==nums[k]: k -= 1 #[0]
+                    while j+1<len(nums) and nums[j+1]==nums[j]: j += 1 #[0]
+                    k -= 1
+                    j += 1
+                    
+        return ans
+
+
+"""
+No Sort.
+Use set to dedupe and check needed.
+
+Time: O(N^2)
+Space: O(N)
+"""
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        ans = set()
+        seen = set()
+        N = len(nums)
+        
+        for i, v1 in enumerate(nums):
+            if v1 in seen: continue
+            seen.add(v1)
+            needed = set()
+            for j, v2 in enumerate(nums[i+1:]):
+                if v2 in needed:
+                    ans.add(tuple(sorted((v1, v2, -v1-v2))))
+                needed.add(-v1-v2)
+        return ans
+
+
+
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        ans = []
+        
+        nums.sort()
+        
+        for i in range(len(nums)):
+            if nums[i]>0: break
+            if i>0 and nums[i]==nums[i-1]: continue
+            
+            j = i+1
+            k = len(nums)-1
+            while j<k:
+                if nums[j]+nums[k]+nums[i]>0:
+                    k -= 1
+                elif nums[j]+nums[k]+nums[i]<0:
+                    j += 1
+                else:
+                    ans.append((nums[i], nums[j], nums[k]))
+                    
+                    while j<k and nums[j+1]==nums[j]: j += 1
+                    while j<k and nums[k-1]==nums[k]: k -= 1
+                    j += 1
+                    k -= 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/4sum.py b/problems/python3/4sum.py
new file mode 100755
index 0000000..4757233
--- /dev/null
+++ b/problems/python3/4sum.py
@@ -0,0 +1,33 @@
+"""
+Time: O(N^(K-1)) -> O(N^3)
+Space: O(K) -> O(1)
+"""
+class Solution:
+    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
+        def kSum(k, start, target):
+            if k>2:
+                for i in range(start, len(nums)-k+1):
+                    if i!=start and nums[i]==nums[i-1]: continue
+                    temp.append(nums[i])
+                    kSum(k-1, i+1, target-nums[i])
+                    temp.pop()
+            else:
+                l, r = start, len(nums)-1
+                
+                while l<r:
+                    if nums[l]+nums[r]>target:
+                        r -= 1
+                    elif nums[l]+nums[r]<target:
+                        l += 1
+                    else:
+                        ans.append(temp+[nums[l], nums[r]])
+                        while l<r and nums[l+1]==nums[l]: l += 1
+                        while l<r and nums[r-1]==nums[r]: r -= 1
+                        l += 1
+                        r -= 1
+                        
+        ans = []
+        temp = []
+        nums.sort()
+        kSum(4, 0, target)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/add-two-numbers.py b/problems/python3/add-two-numbers.py
new file mode 100644
index 0000000..ed9dee4
--- /dev/null
+++ b/problems/python3/add-two-numbers.py
@@ -0,0 +1,19 @@
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+        carry = 0
+        dummy = ListNode()
+        curr = dummy
+        
+        while l1 or l2 or carry:
+            n = (l1.val if l1 else 0) + (l2.val if l2 else 0) + carry
+            if n>=10:
+                curr.next = ListNode(n-10)
+                carry = 1
+            else:
+                curr.next = ListNode(n)
+                carry = 0
+            curr = curr.next
+            if l1: l1 = l1.next
+            if l2: l2 = l2.next
+            
+        return dummy.next
\ No newline at end of file
diff --git a/problems/python3/alien-dictionary.py b/problems/python3/alien-dictionary.py
new file mode 100644
index 0000000..23ca1ee
--- /dev/null
+++ b/problems/python3/alien-dictionary.py
@@ -0,0 +1,36 @@
+class Solution:
+    def alienOrder(self, words: List[str]) -> str:
+        adj = collections.defaultdict(list)
+        inbounds = collections.Counter()
+        q = collections.deque()
+        ans = ''
+        
+        adj = {c: set() for word in words for c in word}
+        for i in range(len(words)-1):
+            w1, w2 = words[i], words[i+1]
+            minLen = min(len(w1), len(w2))
+            if w1[:minLen]==w2[:minLen] and len(w1)>len(w2): return ""
+            
+            for j in range(minLen):
+                if w1[j]!=w2[j]:
+                    adj[w1[j]].add(w2[j])
+                    break
+        
+        for c in adj:
+            for nc in list(adj[c]):
+                inbounds[nc] += 1
+        
+        for c in adj:
+            if inbounds[c]==0: q.append(c)
+        
+        while q:
+            c = q.popleft()
+            
+            ans += c
+            
+            for nc in adj[c]:
+                inbounds[nc] -= 1
+                if inbounds[nc]==0: q.append(nc)
+        
+        return ans if len(ans)==len(adj) else ''
+
diff --git a/problems/python3/balanced-binary-tree.py b/problems/python3/balanced-binary-tree.py
new file mode 100755
index 0000000..65c749e
--- /dev/null
+++ b/problems/python3/balanced-binary-tree.py
@@ -0,0 +1,18 @@
+"""
+Time: O(N)
+Space: O(LogN) Recursion stack. If the tree is balanced.
+"""
+class Solution:
+    def isBalanced(self, root: Optional[TreeNode]) -> bool:
+        def getHeight(node) -> (bool, int):
+            if not node: return True, 0
+            leftIsBalanced, leftHeight = getHeight(node.left)
+            if not leftIsBalanced: return False, 0
+            
+            rightIsBalanced, rightHeight = getHeight(node.right)
+            if not rightIsBalanced: return False, 0
+            
+            return abs(leftHeight-rightHeight)<2, 1+max(leftHeight, rightHeight)
+        
+        isBalanced, h = getHeight(root)
+        return isBalanced
\ No newline at end of file
diff --git a/problems/python3/best-time-to-buy-and-sell-stock-with-cooldown.py b/problems/python3/best-time-to-buy-and-sell-stock-with-cooldown.py
new file mode 100644
index 0000000..06644e3
--- /dev/null
+++ b/problems/python3/best-time-to-buy-and-sell-stock-with-cooldown.py
@@ -0,0 +1,16 @@
+#dp[i][0] := max profit when last action is buy
+#dp[i][1] := max profit when last action is sell
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        if not prices or len(prices)<=1: return 0
+        
+        N = len(prices)
+        dp = [[0, 0] for _ in range(N)]
+        dp[0] = [-prices[0], 0]
+        dp[1][0] = max(-prices[1], -prices[0])
+        dp[1][1] = max(prices[1]+dp[0][0], dp[0][1])
+        
+        for i in range(2, N):
+            dp[i][0] = max(dp[i-2][1]-prices[i], dp[i-1][0])
+            dp[i][1] = max(prices[i]+dp[i-1][0], dp[i-1][1])
+        return max(dp[-1][0], dp[-1][1], 0)
\ No newline at end of file
diff --git a/problems/python3/best-time-to-buy-and-sell-stock.py b/problems/python3/best-time-to-buy-and-sell-stock.py
new file mode 100644
index 0000000..f04ad91
--- /dev/null
+++ b/problems/python3/best-time-to-buy-and-sell-stock.py
@@ -0,0 +1,10 @@
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        minPrice = prices[0]
+        ans = 0
+        
+        for i in range(1, len(prices)):
+            ans = max(ans, prices[i]-minPrice)
+            minPrice = min(prices[i], minPrice)
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/binary-search.py b/problems/python3/binary-search.py
new file mode 100644
index 0000000..2e65a63
--- /dev/null
+++ b/problems/python3/binary-search.py
@@ -0,0 +1,16 @@
+class Solution:
+    def search(self, nums: List[int], target: int) -> int:
+        l = 0
+        r = len(nums)-1
+        
+        while l<=r:
+            m = l + int((r-l)/2)
+            
+            if nums[m]>target:
+                r = m-1
+            elif nums[m]<target:
+                l = m+1
+            else:
+                return m
+        
+        return -1
\ No newline at end of file
diff --git a/problems/python3/binary-tree-level-order-traversal.py b/problems/python3/binary-tree-level-order-traversal.py
new file mode 100755
index 0000000..2ddb875
--- /dev/null
+++ b/problems/python3/binary-tree-level-order-traversal.py
@@ -0,0 +1,39 @@
+"""
+Time: O(N), since we need to go through all the nodes.
+Space: O(LogN) for recursion stack. (if the tree is balanced). This do not take the output into account.
+"""
+class Solution:
+    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        def helper(node, level):
+            if not node: return
+            if level==len(ans): ans.append([])
+            ans[level].append(node.val)
+            helper(node.left, level+1)
+            helper(node.right, level+1)
+        
+        ans = []
+        helper(root, 0)
+        
+        return ans
+
+
+"""
+Time: O(N), since we need to go through all the nodes.
+Space: O(N) for queue.
+"""
+class Solution:
+    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root: return []
+        
+        q = collections.deque([(root, 0)])
+        ans = []
+        
+        while q:
+            node, level = q.popleft()
+            
+            if level==len(ans): ans.append([])
+            ans[level].append(node.val)
+            if node.left: q.append((node.left, level+1))
+            if node.right: q.append((node.right, level+1))
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/binary-tree-maximum-path-sum.py b/problems/python3/binary-tree-maximum-path-sum.py
new file mode 100644
index 0000000..e024506
--- /dev/null
+++ b/problems/python3/binary-tree-maximum-path-sum.py
@@ -0,0 +1,27 @@
+"""
+helper(node) will return the max path sum from the node to the leaf. [0]
+
+Along the way we update the ans. [1]
+The path of ans must pass through one of the node in the tree.
+`node.val+left+right` means the max path sum that pass through the node.
+So after all `helper()` is executed, we tested all the nodes.
+
+Since there are some negative values in the tree, each node can decided not to take the negative path sum into account. [3]
+
+Time: O(N)
+Space: O(LogN) for the recursive stack (if the tree is balanced).
+"""
+class Solution:
+    def maxPathSum(self, root: Optional[TreeNode]) -> int:
+        def helper(node):
+            nonlocal ans
+            
+            if not node: return 0
+            left = max(helper(node.left), 0) #[3]
+            right = max(helper(node.right), 0) #[3]
+            ans = max(ans, node.val+left+right) #[1]
+            return node.val+max(left, right) #[0]
+        
+        ans = float('-inf')
+        helper(root)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/binary-tree-right-side-view.py b/problems/python3/binary-tree-right-side-view.py
new file mode 100755
index 0000000..53b4d13
--- /dev/null
+++ b/problems/python3/binary-tree-right-side-view.py
@@ -0,0 +1,18 @@
+"""
+Time: O(N)
+Space: O(LogN) if the tree is balanced.
+
+BFS is a more intuitive way to do this, but the time complexity will be at least O(N).
+Using recursion instead, will take O(LogN) for the recursion stack size.
+"""
+class Solution:
+    def rightSideView(self, root: Optional[TreeNode]) -> List[int]:
+        def helper(node, level):
+            if not node: return
+            if len(ans)<level: ans.append(node.val)
+            helper(node.right, level+1)
+            helper(node.left, level+1)
+        
+        ans = []
+        helper(root, 1)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/burst-balloons.py b/problems/python3/burst-balloons.py
new file mode 100644
index 0000000..aae31be
--- /dev/null
+++ b/problems/python3/burst-balloons.py
@@ -0,0 +1,28 @@
+"""
+dfs(l, r) return the max coins that we can get at range l to r.
+
+```
+for i in range(l, r+1):
+    dp[(l, r)] = max(dp[(l, r)], nums[l-1]*nums[i]*nums[r+1] + dfs(l, i-1) + dfs(i+1, r))
+```
+Assuming i is the last one we extract, the max coins we can get.
+
+Start from the last becasue, we are not able to track the neighbor if we start from the first we extract.
+
+Time: O(N^3)
+Space: O(N^2)
+"""
+class Solution:
+    def maxCoins(self, nums: List[int]) -> int:
+        def dfs(l, r)->int:
+            if l>r: return 0
+            if (l, r) in dp: return dp[(l, r)]
+            
+            dp[(l, r)] = 0
+            for i in range(l, r+1):
+                dp[(l, r)] = max(dp[(l, r)], nums[l-1]*nums[i]*nums[r+1] + dfs(l, i-1) + dfs(i+1, r))
+            return dp[(l, r)]
+                
+        nums = [1]+nums+[1]
+        dp = {}
+        return dfs(1, len(nums)-2)
\ No newline at end of file
diff --git a/problems/python3/car-fleet.py b/problems/python3/car-fleet.py
new file mode 100644
index 0000000..24ccba8
--- /dev/null
+++ b/problems/python3/car-fleet.py
@@ -0,0 +1,16 @@
+class Solution:
+    def carFleet(self, target: int, position: List[int], speed: List[int]) -> int:
+        N = len(position)
+        
+        timeInfo = []
+        for i in range(N):
+            timeInfo.append((position[i], (target-position[i])/speed[i]))
+        timeInfo.sort(reverse=True)
+        
+        stack = []
+        for _, time in timeInfo:
+            stack.append(time)
+            if len(stack)>=2 and stack[-2]>=stack[-1]:
+                stack.pop()
+        
+        return len(stack)
\ No newline at end of file
diff --git a/problems/python3/cheapest-flights-within-k-stops.py b/problems/python3/cheapest-flights-within-k-stops.py
new file mode 100644
index 0000000..4595802
--- /dev/null
+++ b/problems/python3/cheapest-flights-within-k-stops.py
@@ -0,0 +1,17 @@
+"""
+Bellman-Ford.
+Time: O(KE)
+"""
+class Solution:
+    def findCheapestPrice(self, N: int, flights: List[List[int]], src: int, dst: int, K: int) -> int:
+        prices = {n:float('inf') for n in range(N)}
+        prices[src] = 0
+        
+        for k in range(K+1):
+            temp = prices.copy()
+            for source, destination, price in flights:
+                if prices[source]==float('inf'): continue
+                if prices[source]+price<temp[destination]:
+                    temp[destination] = prices[source]+price
+            prices = temp
+        return prices[dst] if prices[dst]!=float('inf') else -1
\ No newline at end of file
diff --git a/problems/python3/climbing-stairs.py b/problems/python3/climbing-stairs.py
new file mode 100644
index 0000000..5dc281b
--- /dev/null
+++ b/problems/python3/climbing-stairs.py
@@ -0,0 +1,10 @@
+class Solution:
+    def climbStairs(self, N: int) -> int:
+        dp = [0]*(N+1)
+        dp[0] = 1
+        for i in range(len(dp)):
+            if i-1>=0:
+                dp[i] += dp[i-1]
+            if i-2>=0:
+                dp[i] += dp[i-2]
+        return dp[-1]
\ No newline at end of file
diff --git a/problems/python3/clone-graph.py b/problems/python3/clone-graph.py
new file mode 100644
index 0000000..9a6765f
--- /dev/null
+++ b/problems/python3/clone-graph.py
@@ -0,0 +1,16 @@
+class Solution:
+    def cloneGraph(self, start: 'Node') -> 'Node':
+        def dfs(node):
+            if node in clones: return clones[node]
+            
+            copy = Node(node.val)
+            clones[node] = copy
+            
+            for neighbor in node.neighbors:
+                copy.neighbors.append(dfs(neighbor))
+                
+            return clones[node]
+        if not start: return start
+        clones = {}
+        dfs(start)
+        return clones[start]
\ No newline at end of file
diff --git a/problems/python3/coin-change-ii.py b/problems/python3/coin-change-ii.py
new file mode 100644
index 0000000..8b4ad26
--- /dev/null
+++ b/problems/python3/coin-change-ii.py
@@ -0,0 +1,12 @@
+class Solution:
+    def change(self, amount: int, coins: List[int]) -> int:
+        dp = [0]*(amount+1)
+        dp[0] = 1
+        
+        coins.sort()
+        
+        for coin in coins:
+            for a in range(1, amount+1):
+                if a-coin<0: continue
+                dp[a] += dp[a-coin]
+        return dp[-1]
\ No newline at end of file
diff --git a/problems/python3/coin-change.py b/problems/python3/coin-change.py
new file mode 100644
index 0000000..19571e4
--- /dev/null
+++ b/problems/python3/coin-change.py
@@ -0,0 +1,14 @@
+class Solution:
+    def coinChange(self, coins: List[int], amount: int) -> int:
+        dp = [float('inf')]*(amount+1)
+        dp[0] = 0
+        for coin in coins:
+            if coin<len(dp): dp[coin] = 1
+        
+        coins.sort()
+        
+        for i in range(1, amount+1):
+            for coin in coins:
+                if i-coin<0: break
+                dp[i] = min(dp[i], dp[i-coin]+1)
+        return dp[amount] if dp[amount]!=float('inf') else -1
\ No newline at end of file
diff --git a/problems/python3/combination-sum-ii.py b/problems/python3/combination-sum-ii.py
new file mode 100644
index 0000000..c9f8957
--- /dev/null
+++ b/problems/python3/combination-sum-ii.py
@@ -0,0 +1,22 @@
+class Solution:
+    def combinationSum2(self, candidates: List[int], target: int) -> List[List[int]]:
+        def helper(i, target):
+            if target==0:
+                ans.append(combination.copy())
+                return
+            
+            if i==len(candidates) or candidates[i]>target:
+                return
+            
+            combination.append(candidates[i])
+            helper(i+1, target-candidates[i])
+            combination.pop()
+            
+            while i+1<len(candidates) and candidates[i]==candidates[i+1]: i += 1
+            helper(i+1, target)
+        
+        ans = []
+        combination = []
+        candidates.sort()
+        helper(0, target)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/combination-sum.py b/problems/python3/combination-sum.py
new file mode 100644
index 0000000..256cefa
--- /dev/null
+++ b/problems/python3/combination-sum.py
@@ -0,0 +1,29 @@
+"""
+Time: O(N^K), assuming the number of element in the combination that sums up to "target" is K.
+For each element, there is N choices.
+Which means the number of combination is N*N*N... for K times.
+~= O(N^K)
+
+Space: O(K)
+
+K is approximate to target/min(candidates).
+"""
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        def helper(i, currSum, target):
+            if currSum>target:
+                return
+            
+            if currSum==target:
+                ans.append(combination.copy())
+                return
+            
+            for j in range(i, len(candidates)):
+                combination.append(candidates[j])
+                helper(j, currSum+candidates[j], target)
+                combination.pop()
+                
+        ans = []
+        combination = []
+        helper(0, 0, target)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/construct-binary-tree-from-preorder-and-inorder-traversal.py b/problems/python3/construct-binary-tree-from-preorder-and-inorder-traversal.py
new file mode 100644
index 0000000..eb7f19c
--- /dev/null
+++ b/problems/python3/construct-binary-tree-from-preorder-and-inorder-traversal.py
@@ -0,0 +1,34 @@
+"""
+preorder: [root][left][right]
+inorder:  [left][root][right]
+
+[0]
+As you can see, the root is always located at the first index of the preorder list.
+
+[2]
+Using the same logic, we can recursively get the left and right node
+Inorder to do that, we need to define left and right nodes' range.
+And we can get the length of the left subtree by locating the index of the root in the in order list. [1]
+i and j is the startIndex and the (endIndex+1) of the preorder list.
+k and l is the startIndex and the (endIndex+1) of the inorder list.
+
+Time: O(N)
+Space: O(N)
+"""
+class Solution:
+    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        def helper(i, j, k, l):
+            if i==j or k==l: return None
+            
+            root = TreeNode(preorder[i]) #[0]
+            
+            rootInorderIndex = getInorderIndex[root.val] #[1]
+            leftLen = rootInorderIndex-k
+            
+            root.left = helper(i+1, i+1+leftLen, k, k+leftLen) #[2]
+            root.right = helper(i+1+leftLen, j, rootInorderIndex+1, l)
+            return root
+        
+        getInorderIndex = {}
+        for i, v in enumerate(inorder): getInorderIndex[v] = i
+        return helper(0, len(preorder), 0, len(inorder))
\ No newline at end of file
diff --git a/problems/python3/container-with-most-water.py b/problems/python3/container-with-most-water.py
new file mode 100755
index 0000000..a560441
--- /dev/null
+++ b/problems/python3/container-with-most-water.py
@@ -0,0 +1,42 @@
+"""
+Time: O(N)
+Space: O(1)
+
+i and j starts from the leftest and rightest. Move the one that has less height.
+
+Why we won't miss any i and j?
+For example, currently i is heigher than j.
+If between i~j, there are no height that is larger or equal to i, than since the area is `min(height[i], height[j]) * (j-i)`, you cannot find any area that is larger than the current one.
+If between i~j, there is a height that is larger or equal to i, j will on it, and it will be tested.
+Thus, given any i and j, any other future i and j that have the potential of forming larger area will be tested.
+"""
+class Solution:
+    def maxArea(self, height: List[int]) -> int:
+        i = 0
+        j = len(height)-1
+        ans = 0
+        
+        while i<j:
+            ans = max(ans, min(height[i], height[j]) * (j-i))
+            if height[i]>height[j]:
+                j -= 1
+            else:
+                i += 1
+        return ans
+
+
+
+class Solution:
+    def maxArea(self, height: List[int]) -> int:
+        i = 0
+        j = len(height)-1
+        ans = 0
+        
+        while i<j:
+            ans = max(ans, min(height[i], height[j]) * abs(i-j))
+            
+            if height[i]>height[j]:
+                j -= 1
+            else:
+                i += 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/contains-duplicate.py b/problems/python3/contains-duplicate.py
new file mode 100644
index 0000000..7849e84
--- /dev/null
+++ b/problems/python3/contains-duplicate.py
@@ -0,0 +1,7 @@
+class Solution:
+    def containsDuplicate(self, nums: List[int]) -> bool:
+        seen = set()
+        for num in nums:
+            if num in seen: return True
+            seen.add(num)
+        return False
\ No newline at end of file
diff --git a/problems/python3/copy-list-with-random-pointer.py b/problems/python3/copy-list-with-random-pointer.py
new file mode 100644
index 0000000..4436b78
--- /dev/null
+++ b/problems/python3/copy-list-with-random-pointer.py
@@ -0,0 +1,34 @@
+"""
+Time: O(N)
+Space: O(1)
+
+The easiest way would be maintaining a hash map for original node to the copy and the other way around. Then the rest is easy.
+But this will take us O(N) of extra space.
+
+Two pass solution with constant space.
+First pass.
+Create a copy of the original and store it inside "random".
+The copy points to original's next and original's random.
+
+Second pass.
+Iterate through the nodes again.
+This time we adjust the copy to point to the other copies.
+"""
+class Solution:
+    def copyRandomList(self, head: 'Optional[Node]') -> 'Optional[Node]':
+        if not head: return head
+        
+        node = head
+        while node:
+            copy = Node(node.val, node.next, node.random)
+            node.random = copy
+            node = node.next
+        
+        node = head
+        while node:
+            newNode = node.random
+            if node.next: newNode.next = node.next.random
+            if newNode.random: newNode.random = newNode.random.random
+            node = node.next
+        
+        return head.random
\ No newline at end of file
diff --git a/problems/python3/count-good-nodes-in-binary-tree.py b/problems/python3/count-good-nodes-in-binary-tree.py
new file mode 100755
index 0000000..2288d12
--- /dev/null
+++ b/problems/python3/count-good-nodes-in-binary-tree.py
@@ -0,0 +1,16 @@
+"""
+Time: O(N)
+Space: O(LogN) for recursion stack if the tree is balanced.
+"""
+class Solution:
+    def goodNodes(self, root: TreeNode) -> int:
+        def helper(node, maxVal):
+            nonlocal count            
+            if not node: return
+            if node.val>=maxVal: count += 1
+            helper(node.left, max(maxVal, node.val))
+            helper(node.right, max(maxVal, node.val))
+        
+        count = 0
+        helper(root, float('-inf'))
+        return count
\ No newline at end of file
diff --git a/problems/python3/counting-bits.py b/problems/python3/counting-bits.py
new file mode 100644
index 0000000..7325596
--- /dev/null
+++ b/problems/python3/counting-bits.py
@@ -0,0 +1,9 @@
+class Solution:
+    def countBits(self, n: int) -> List[int]:
+        ans = [0]*(n+1)
+        offset = 1
+        
+        for i in range(1, n+1):
+            if offset*2==i: offset = offset*2
+            ans[i] = 1+ans[i-offset]
+        return ans
\ No newline at end of file
diff --git a/problems/python3/course-schedule-ii.py b/problems/python3/course-schedule-ii.py
new file mode 100644
index 0000000..00725b4
--- /dev/null
+++ b/problems/python3/course-schedule-ii.py
@@ -0,0 +1,30 @@
+#Topological Sort
+class Solution:
+    def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
+        sortedCourse = []
+        G = collections.defaultdict(list) #graph
+        inbounds = collections.Counter()
+        q = collections.deque()
+        
+        #build graph
+        for c1, c2 in prerequisites:
+            G[c2].append(c1)
+            inbounds[c1] += 1
+        
+        #add the starting point to the q. (the ones that have 0 inbounds)
+        for course in range(numCourses):
+            if inbounds[course]==0: q.append(course)
+        
+        #add the course that have 0 inbounds to the sortedCourse.
+        #after that, imagine we remove it from the graph, so nextCourse inbound will -1
+        #add to q if nextCourse have 0 inbounds
+        while q:
+            course = q.popleft()
+            
+            sortedCourse.append(course)
+            
+            for nextCourse in G[course]:
+                inbounds[nextCourse] -= 1
+                if inbounds[nextCourse]==0: q.append(nextCourse)
+        
+        return sortedCourse if len(sortedCourse)==numCourses else []
\ No newline at end of file
diff --git a/problems/python3/course-schedule.py b/problems/python3/course-schedule.py
new file mode 100644
index 0000000..f52d07d
--- /dev/null
+++ b/problems/python3/course-schedule.py
@@ -0,0 +1,23 @@
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+        sortedCourse = []
+        inbounds = collections.Counter()
+        G = collections.defaultdict(list)
+        q = collections.deque()
+        
+        for c1, c2 in prerequisites:
+            G[c2].append(c1)
+            inbounds[c1] += 1
+        
+        for c in range(numCourses):
+            if inbounds[c]==0: q.append(c)
+
+        while q:
+            c = q.popleft()
+            
+            for c2 in G[c]:
+                inbounds[c2] -= 1
+                if inbounds[c2]==0: q.append(c2)    
+            sortedCourse.append(c)
+        
+        return len(sortedCourse)==numCourses
\ No newline at end of file
diff --git a/problems/python3/daily-temperatures.py b/problems/python3/daily-temperatures.py
new file mode 100644
index 0000000..d6e4c98
--- /dev/null
+++ b/problems/python3/daily-temperatures.py
@@ -0,0 +1,12 @@
+class Solution:
+    def dailyTemperatures(self, temperatures: List[int]) -> List[int]:
+        ans = [0]*len(temperatures)
+        stack = []
+        
+        for i, temp in enumerate(temperatures):
+            while stack and stack[-1][0]<temp:
+                prevTemp, prevIndex = stack.pop()
+                ans[prevIndex] = i-prevIndex
+            stack.append((temp, i))
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/decode-ways.py b/problems/python3/decode-ways.py
new file mode 100644
index 0000000..3ac6c2d
--- /dev/null
+++ b/problems/python3/decode-ways.py
@@ -0,0 +1,14 @@
+"""
+dp[i] := up until s[:i] how many possibility?
+"""
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        mapping = set([str(n) for n in range(1, 27)])
+        N = len(s)
+        dp = [0]*(N+1)
+        dp[0] = 1
+        
+        for i in range(1, N+1):
+            if i-1>=0 and s[i-1] in mapping: dp[i] += dp[i-1]
+            if i-2>=0 and s[i-2:i] in mapping: dp[i] += dp[i-2]
+        return dp[-1]
\ No newline at end of file
diff --git a/problems/python3/design-add-and-search-words-data-structure.py b/problems/python3/design-add-and-search-words-data-structure.py
new file mode 100644
index 0000000..52effc8
--- /dev/null
+++ b/problems/python3/design-add-and-search-words-data-structure.py
@@ -0,0 +1,33 @@
+class WordDictionary:
+
+    def __init__(self):
+        self.root = {}
+
+    def addWord(self, word: str) -> None:
+        node = self.root
+        for c in word:
+            if c not in node: node[c] = {}
+            node = node[c]
+        node['$'] = {} #'.' means the end of the word
+
+    def search(self, word: str) -> bool:
+        def helper(node, i):
+            if i==len(word): return '$' in node
+                
+            if word[i]=='.':
+                for c in node:
+                    if helper(node[c], i+1): return True
+                return False
+            else:
+                if word[i] not in node: return False
+                return helper(node[word[i]], i+1)
+        return helper(self.root, 0)
+            
+        
+        
+        
+        
+    
+    
+    
+    
\ No newline at end of file
diff --git a/problems/python3/design-twitter.py b/problems/python3/design-twitter.py
new file mode 100644
index 0000000..a2781ae
--- /dev/null
+++ b/problems/python3/design-twitter.py
@@ -0,0 +1,49 @@
+class Twitter:
+
+    def __init__(self):
+        self.followData = collections.defaultdict(set)
+        self.tweetData = collections.defaultdict(list)
+        self.count = 0
+        
+
+    def postTweet(self, userId: int, tweetId: int) -> None:
+        self.tweetData[userId].append((self.count, tweetId))
+        self.count -= 1
+
+    
+    def getNewsFeed(self, userId: int) -> List[int]:
+        newsFeed = []
+        h = []
+        
+        self.followData[userId].add(userId)
+        for followeeId in self.followData[userId]:
+            if followeeId not in self.tweetData: continue
+            index = len(self.tweetData[followeeId])-1
+            count, tweetId = self.tweetData[followeeId][index]
+            h.append((count, tweetId, followeeId, index-1))
+        heapq.heapify(h)
+        
+        while h and len(newsFeed)<10:
+            _, tweetId, userId, index = heapq.heappop(h)
+            newsFeed.append(tweetId)
+            if index>=0:
+                count, tweetId2 = self.tweetData[userId][index]
+                heapq.heappush(h, (count, tweetId2, userId, index-1))
+        return newsFeed
+        
+        
+
+    def follow(self, followerId: int, followeeId: int) -> None:
+        self.followData[followerId].add(followeeId)
+
+    def unfollow(self, followerId: int, followeeId: int) -> None:
+        if followerId not in self.followData: return
+        self.followData[followerId].remove(followeeId)
+
+
+# Your Twitter object will be instantiated and called as such:
+# obj = Twitter()
+# obj.postTweet(userId,tweetId)
+# param_2 = obj.getNewsFeed(userId)
+# obj.follow(followerId,followeeId)
+# obj.unfollow(followerId,followeeId)
\ No newline at end of file
diff --git a/problems/python3/detect-squares.py b/problems/python3/detect-squares.py
new file mode 100644
index 0000000..af86946
--- /dev/null
+++ b/problems/python3/detect-squares.py
@@ -0,0 +1,22 @@
+class DetectSquares:
+
+    def __init__(self):
+        self.store = collections.Counter()
+
+    def add(self, point: List[int]) -> None:
+        self.store[tuple(point)] += 1
+
+    def count(self, point: List[int]) -> int:
+        x, y = point
+        ans = 0
+        
+        for dx, dy in self.store:
+            if abs(x-dx)!=abs(y-dy) or x==dx or y==dy: continue
+            ans += self.store[(dx, dy)]*self.store[(dx, y)]*self.store[(x, dy)]
+        return ans
+
+
+# Your DetectSquares object will be instantiated and called as such:
+# obj = DetectSquares()
+# obj.add(point)
+# param_2 = obj.count(point)
\ No newline at end of file
diff --git a/problems/python3/diameter-of-binary-tree.py b/problems/python3/diameter-of-binary-tree.py
new file mode 100755
index 0000000..1f9704d
--- /dev/null
+++ b/problems/python3/diameter-of-binary-tree.py
@@ -0,0 +1,17 @@
+"""
+Time: O(N)
+Space: O(LogN) for the recursion stack. If the tree is balanced.
+"""
+class Solution:
+    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
+        def helper(node):
+            if not node: return 0
+            l = helper(node.left)
+            r = helper(node.right)
+            self.ans = max(self.ans, 1+l+r)
+            return 1+max(l, r)
+        
+        self.ans = 0
+        
+        helper(root)
+        return self.ans-1
\ No newline at end of file
diff --git a/problems/python3/distinct-subsequences.py b/problems/python3/distinct-subsequences.py
new file mode 100644
index 0000000..d84fcf6
--- /dev/null
+++ b/problems/python3/distinct-subsequences.py
@@ -0,0 +1,17 @@
+class Solution:
+    def numDistinct(self, s: str, t: str) -> int:
+        def dfs(i, j):
+            if (i, j) in visited: return visited[(i, j)]
+            
+            if j>=len(t): return 1
+            if i>=len(s): return 0
+            
+            if s[i]==t[j]:
+                visited[(i, j)] = dfs(i+1, j+1)+dfs(i+1, j)
+            else:
+                visited[(i, j)] = dfs(i+1, j)
+            return visited[(i, j)]
+            
+        visited = {}
+        dfs(0, 0)
+        return dfs(0, 0)
\ No newline at end of file
diff --git a/problems/python3/edit-distance.py b/problems/python3/edit-distance.py
new file mode 100644
index 0000000..c8fa8f4
--- /dev/null
+++ b/problems/python3/edit-distance.py
@@ -0,0 +1,37 @@
+"""
+dfs(i, j)
+i being the unprocessed index in word1.
+j, word2.
+
+MAIN LOGIC:
+if word1[i]==word2[j], no operation need, return dfs(i+1, j+1)
+if not, need 1 operation, so
+replace: dfs(i+1, j+1)
+insert: dfs(i, j+1)
+delete: dfs(i+1, j)
+
+BASE CASE:
+If both string are empty (i==N and j==M), no operation needed.
+If one string are empty, then the remain operation is the length of the non-empty one.
+"""
+class Solution:
+    def minDistance(self, word1: str, word2: str) -> int:
+        N = len(word1)
+        M = len(word2)
+        def dfs(i, j)->int:
+            if i==N and j==M: return 0
+            if i==N: return M-j
+            if j==M: return N-i
+            
+            if (i, j) in history:
+                return history[(i, j)]
+                
+            if word1[i]==word2[j]:
+                history[(i, j)] = dfs(i+1, j+1)
+            else:
+                history[(i, j)] = 1+min(dfs(i+1, j+1), dfs(i+1, j), dfs(i, j+1))
+            
+            return history[(i, j)]
+        
+        history = {}
+        return dfs(0, 0)
\ No newline at end of file
diff --git a/problems/python3/encode-and-decode-strings.py b/problems/python3/encode-and-decode-strings.py
new file mode 100644
index 0000000..f38c4e4
--- /dev/null
+++ b/problems/python3/encode-and-decode-strings.py
@@ -0,0 +1,23 @@
+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        output = ''
+        for string in strs:
+            output += str(len(string))+'#'+string
+        return output
+        
+
+    def decode(self, s: str) -> List[str]:
+        output = []
+        i = 0
+        
+        while i<len(s):
+            j = i+1
+            
+            while s[j]!='#': j += 1
+                    
+            l = int(s[i:j])
+            string = s[j+1:j+1+l]
+            output.append(string)
+            i = j+1+l
+            
+        return output
\ No newline at end of file
diff --git a/problems/python3/evaluate-reverse-polish-notation.py b/problems/python3/evaluate-reverse-polish-notation.py
new file mode 100644
index 0000000..bbaf62e
--- /dev/null
+++ b/problems/python3/evaluate-reverse-polish-notation.py
@@ -0,0 +1,22 @@
+class Solution:
+    def evalRPN(self, tokens: List[str]) -> int:
+        operators = set(['+', '-', '*', '/'])
+        stack = []
+        
+        for c in tokens:
+            if c in operators:
+                n2 = stack.pop()
+                n1 = stack.pop()
+                
+                if c=='+':
+                    stack.append(n1+n2)
+                elif c=='-':
+                    stack.append(n1-n2)
+                elif c=='*':
+                    stack.append(n1*n2)
+                elif c=='/':
+                    stack.append(int(n1/n2))
+            else:
+                stack.append(int(c))
+        
+        return stack.pop()
\ No newline at end of file
diff --git a/problems/python3/find-median-from-data-stream.py b/problems/python3/find-median-from-data-stream.py
new file mode 100644
index 0000000..f83ee40
--- /dev/null
+++ b/problems/python3/find-median-from-data-stream.py
@@ -0,0 +1,32 @@
+class MedianFinder:
+
+    def __init__(self):
+        self.large = [] #store nums larger or equal to the median
+        self.small = [] #store nums samaller to the median
+
+    def addNum(self, num: int) -> None:
+        if not self.large and not self.small:
+            heapq.heappush(self.large, num)
+        elif num>=self.findMedian():
+            heapq.heappush(self.large, num)
+            self.balance()
+        else:
+            heapq.heappush(self.small, -num)
+            self.balance()
+    
+    def balance(self) -> None:
+        #make the length of two heaps as even as posible
+        if len(self.large)>len(self.small)+1:
+            num = heapq.heappop(self.large)
+            heapq.heappush(self.small, -num)
+        
+        if len(self.small)>len(self.large):
+            num = -heapq.heappop(self.small)
+            heapq.heappush(self.large, num)
+    
+
+    def findMedian(self) -> float:
+        if (len(self.large)+len(self.small))%2==0:
+            return (self.large[0]-self.small[0])/2
+        else:
+            return self.large[0]
\ No newline at end of file
diff --git a/problems/python3/find-minimum-in-rotated-sorted-array.py b/problems/python3/find-minimum-in-rotated-sorted-array.py
new file mode 100644
index 0000000..c9af305
--- /dev/null
+++ b/problems/python3/find-minimum-in-rotated-sorted-array.py
@@ -0,0 +1,22 @@
+"""
+Usually the binary search problem, the hard part is to clear all the edge cases.
+What will be the edge case? It will be when the recursive function executed at the deepest level. Usually the list with only one or two element.
+So I will suggest before submit, try out the case like [0,1] or [1,0] to make sure it will not run unstop.
+"""
+class Solution:
+    def findMin(self, nums: List[int]) -> int:
+        def helper(l, r):
+            nonlocal ans
+            if l>r: return
+            
+            m = l + int((r-l)/2)
+            if nums[l]<=nums[m]:
+                ans = min(ans, nums[l])
+                helper(m+1, r)
+            else:
+                ans = min(ans, nums[m+1])
+                helper(l, m)
+        
+        ans = float('inf')
+        helper(0, len(nums)-1)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/find-the-duplicate-number.py b/problems/python3/find-the-duplicate-number.py
new file mode 100644
index 0000000..41636df
--- /dev/null
+++ b/problems/python3/find-the-duplicate-number.py
@@ -0,0 +1,16 @@
+class Solution:
+    def findDuplicate(self, nums: List[int]) -> int:
+        head = nums[0]
+        slow = head
+        fast = head
+        
+        while True:
+            slow = nums[slow]
+            fast = nums[nums[fast]]
+            if slow==fast: break
+        
+        slow = head
+        while slow!=fast:
+            slow = nums[slow]
+            fast = nums[fast]
+        return slow
\ No newline at end of file
diff --git a/problems/python3/gas-station.py b/problems/python3/gas-station.py
new file mode 100644
index 0000000..017eaa6
--- /dev/null
+++ b/problems/python3/gas-station.py
@@ -0,0 +1,23 @@
+"""
+First thing, you must understand that if sum(gas)>=sum(cost) there must be an answer. Guaranteed.
+If we know there is an answer, we can simply test all the index.
+If the currGas ever drops to below 0, it means that we need to switch a "start".
+
+Time: O(N)
+Space: O(1)
+"""
+class Solution:
+    def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
+        if sum(gas)<sum(cost): return -1
+        
+        start = 0
+        currGas = 0
+        
+        for i in range(len(gas)):
+            currGas += gas[i]-cost[i]
+            
+            if currGas<0:
+                currGas = 0
+                start = i+1
+        
+        return start
\ No newline at end of file
diff --git a/problems/python3/generate-parentheses.py b/problems/python3/generate-parentheses.py
new file mode 100644
index 0000000..9b96ef9
--- /dev/null
+++ b/problems/python3/generate-parentheses.py
@@ -0,0 +1,20 @@
+class Solution:
+    def generateParenthesis(self, n: int) -> List[str]:
+        def helper(parentheses, left, right):
+            if len(parentheses)==n*2:
+                ans.append("".join(parentheses))
+                return
+            
+            if left<n:
+                parentheses.append('(')
+                helper(parentheses, left+1, right)
+                parentheses.pop()
+            
+            if left>right:
+                parentheses.append(')')
+                helper(parentheses, left, right+1)
+                parentheses.pop()
+        
+        ans = []
+        helper([], 0, 0)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/graph-valid-tree.py b/problems/python3/graph-valid-tree.py
new file mode 100644
index 0000000..2168e9a
--- /dev/null
+++ b/problems/python3/graph-valid-tree.py
@@ -0,0 +1,33 @@
+class Solution:
+    def validTree(self, N: int, edges: List[List[int]]) -> bool:
+        def union(n1, n2) -> bool:
+            p1 = find(n1)
+            p2 = find(n2)
+            
+            if p1==p2:
+                return False
+            elif p1<p2:
+                parents[p2] = p1
+            else:
+                parents[p1] = p2
+                
+            return True
+            
+        
+        def find(n) -> int:
+            p = parents[n]
+            while p!=parents[p]: p = find(p)
+            parents[n] = p
+            return p
+        
+        parents = [n for n in range(N)]
+
+        for n1, n2 in edges:
+            if not union(n1, n2): return False
+        
+        #check if all node trace back to the same root
+        root = find(0)
+        for n in range(1, N):
+            if root!=find(n): return False
+        
+        return True
\ No newline at end of file
diff --git a/problems/python3/group-anagrams.py b/problems/python3/group-anagrams.py
new file mode 100644
index 0000000..7430f48
--- /dev/null
+++ b/problems/python3/group-anagrams.py
@@ -0,0 +1,23 @@
+class Solution:
+    def groupAnagrams(self, strs: List[str]) -> List[List[str]]:
+        def normalize(string: str) -> str:
+            counter = collections.Counter()
+            ans = ''
+            
+            for c in string:
+                counter[c] += 1
+            
+            for c in 'abcdefghijklmnopqrstuvwxyz':
+                if counter[c]>0:
+                    ans += c+str(counter[c])
+            return ans
+                    
+        group = collections.defaultdict(list)
+        ans = []
+        
+        for string in strs:
+            group[normalize(string)].append(string)
+        
+        for normalizedString in group:
+            ans.append(group[normalizedString])    
+        return ans
\ No newline at end of file
diff --git a/problems/python3/hand-of-straights.py b/problems/python3/hand-of-straights.py
new file mode 100644
index 0000000..f20c32d
--- /dev/null
+++ b/problems/python3/hand-of-straights.py
@@ -0,0 +1,18 @@
+class Solution:
+    def isNStraightHand(self, hand: List[int], groupSize: int) -> bool:
+        if len(hand)%groupSize!=0: return False
+        
+        counter = collections.Counter(hand)
+        h = list(counter.keys())
+        heapq.heapify(h)
+        
+        while h:
+            minNum = h[0]
+            for n in range(minNum, minNum+groupSize):
+                if counter[n]<=0: return False
+                counter[n] -= 1
+                if counter[n]==0:
+                    if h[0]!=n: return False
+                    heapq.heappop(h)
+        
+        return True
\ No newline at end of file
diff --git a/problems/python3/happy-number.py b/problems/python3/happy-number.py
new file mode 100644
index 0000000..7756197
--- /dev/null
+++ b/problems/python3/happy-number.py
@@ -0,0 +1,17 @@
+class Solution:
+    def isHappy(self, n: int) -> bool:
+        def digitSquare(n) -> int:
+            ans = 0
+            while n>0:
+                ans += (n%10)**2
+                n = n//10
+            return ans
+
+        visited = set()
+        visited.add(1)
+
+        while n not in visited:
+            visited.add(n)
+            n = digitSquare(n)
+
+        return n==1
diff --git a/problems/python3/house-robber-ii.py b/problems/python3/house-robber-ii.py
new file mode 100644
index 0000000..1ed9b82
--- /dev/null
+++ b/problems/python3/house-robber-ii.py
@@ -0,0 +1,18 @@
+class Solution:
+    def rob(self, nums: List[int]) -> int:
+        if len(nums)<=1: return max(nums)
+        
+        N = len(nums)
+        dp = [[0, 0] for _ in range(N)]
+        dp[0][0] = nums[0]
+        
+        for i in range(1, N):
+            dp[i][0] = nums[i]+dp[i-1][1]
+            dp[i][1] = max(dp[i-1])
+        
+        dp2 = [[0, 0] for _ in range(N)]
+        for i in range(1, N):
+            dp2[i][0] = nums[i]+dp2[i-1][1]
+            dp2[i][1] = max(dp2[i-1])
+        
+        return max(dp[-1][1], dp2[-1][0])
\ No newline at end of file
diff --git a/problems/python3/house-robber.py b/problems/python3/house-robber.py
new file mode 100644
index 0000000..fbd86a5
--- /dev/null
+++ b/problems/python3/house-robber.py
@@ -0,0 +1,17 @@
+"""
+Time: O(N)
+Space: O(N), can reduece to O(1).
+
+dp[i][0] := max revenue if house i robbed
+dp[i][1] := max revenue if house i not robbed
+"""
+class Solution:
+    def rob(self, nums: List[int]) -> int:
+        N = len(nums)
+        dp = [[0, 0] for _ in range(N)]
+        dp[0][0] = nums[0]
+        
+        for i in range(1, N):
+            dp[i][0] = nums[i]+dp[i-1][1]
+            dp[i][1] = max(dp[i-1])
+        return max(dp[-1])
\ No newline at end of file
diff --git a/problems/python3/implement-trie-prefix-tree.py b/problems/python3/implement-trie-prefix-tree.py
new file mode 100644
index 0000000..1118e97
--- /dev/null
+++ b/problems/python3/implement-trie-prefix-tree.py
@@ -0,0 +1,26 @@
+class Trie:
+
+    def __init__(self):
+        self.root = {}
+
+    def insert(self, word: str) -> None:
+        node = self.root
+        for c in word:
+            if c not in node: node[c] = {}
+            node = node[c]
+        node['.'] = {} #'.' means the end of the word
+        
+        
+    def search(self, word: str) -> bool:
+        node = self.root
+        for c in word:
+            if c not in node: return False
+            node = node[c]
+        return '.' in node
+
+    def startsWith(self, prefix: str) -> bool:
+        node = self.root
+        for c in prefix:
+            if c not in node: return False
+            node = node[c]
+        return True
diff --git a/problems/python3/insert-interval.py b/problems/python3/insert-interval.py
new file mode 100644
index 0000000..667ba04
--- /dev/null
+++ b/problems/python3/insert-interval.py
@@ -0,0 +1,23 @@
+class Solution:
+    def insert(self, intervals: List[List[int]], newInterval: List[int]) -> List[List[int]]:
+        ans = []
+        i = 0
+        
+        #add intervals before newInterval
+        while i<len(intervals) and intervals[i][1]<newInterval[0]:
+            ans.append(intervals[i])
+            i += 1
+        
+        #add newInterval and merge the overlapped
+        ans.append(newInterval)
+        while i<len(intervals) and intervals[i][0]<=ans[-1][1]:
+            ans[-1][0] = min(ans[-1][0], intervals[i][0])
+            ans[-1][1] = max(ans[-1][1], intervals[i][1])
+            i += 1
+        
+        #add intervals after newInterval
+        while i<len(intervals):
+            ans.append(intervals[i])
+            i += 1
+            
+        return ans
\ No newline at end of file
diff --git a/problems/python3/interleaving-string.py b/problems/python3/interleaving-string.py
new file mode 100644
index 0000000..76f9837
--- /dev/null
+++ b/problems/python3/interleaving-string.py
@@ -0,0 +1,13 @@
+class Solution:
+    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:
+        def dfs(i, j):
+            if (i, j) in history: return False
+            if i+j==len(s3): return True
+            if i<len(s1) and s3[i+j]==s1[i] and dfs(i+1, j): return True
+            if j<len(s2) and s3[i+j]==s2[j] and dfs(i, j+1): return True
+            history.add((i, j))
+            return False
+        
+        if len(s1)+len(s2)!=len(s3): return False
+        history = set() #store the failed cases
+        return dfs(0, 0)
\ No newline at end of file
diff --git a/problems/python3/invert-binary-tree.py b/problems/python3/invert-binary-tree.py
new file mode 100755
index 0000000..d69daec
--- /dev/null
+++ b/problems/python3/invert-binary-tree.py
@@ -0,0 +1,36 @@
+"""
+Recursive
+Time: O(N)
+Space: O(LogN)
+"""
+class Solution:
+    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root: return root
+        left = root.left
+        right = root.right
+        root.left = self.invertTree(right)
+        root.right = self.invertTree(left)
+        return root
+
+
+"""
+Iterative
+Time: O(N)
+Space: O(N)
+"""
+class Solution:
+    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root: return root
+        q = collections.deque([root])
+        
+        while q:
+            node = q.popleft()
+            left = node.left
+            right = node.right
+            node.right = left
+            node.left = right
+            
+            if node.left: q.append(node.left)
+            if node.right: q.append(node.right)
+        
+        return root
\ No newline at end of file
diff --git a/problems/python3/jump-game-ii.py b/problems/python3/jump-game-ii.py
new file mode 100644
index 0000000..35be09f
--- /dev/null
+++ b/problems/python3/jump-game-ii.py
@@ -0,0 +1,22 @@
+"""
+l and r is the index range we can reach within "steps".
+So while r can not reach the end (`r<len(nums)-1`), we keep update l, r and steps.
+
+Time: O(N)
+Space: O(1)
+"""
+class Solution:
+    def jump(self, nums: List[int]) -> int:
+        l = r = 0
+        steps = 0
+        
+        while r<len(nums)-1:
+            farest = float('-inf')
+            for i in range(l, r+1):
+                farest = max(farest, i+nums[i])
+            
+            l = r+1
+            r = farest
+            steps += 1
+            
+        return steps
\ No newline at end of file
diff --git a/problems/python3/jump-game.py b/problems/python3/jump-game.py
new file mode 100644
index 0000000..cbf2c89
--- /dev/null
+++ b/problems/python3/jump-game.py
@@ -0,0 +1,10 @@
+class Solution:
+    def canJump(self, nums: List[int]) -> bool:
+        maxIndex = 0
+        
+        for i, num in enumerate(nums):
+            if maxIndex<i:
+                return False
+            maxIndex = max(maxIndex, i+num)
+            
+        return True
\ No newline at end of file
diff --git a/problems/python3/k-closest-points-to-origin.py b/problems/python3/k-closest-points-to-origin.py
new file mode 100644
index 0000000..8f08e89
--- /dev/null
+++ b/problems/python3/k-closest-points-to-origin.py
@@ -0,0 +1,10 @@
+class Solution:
+    def kClosest(self, points: List[List[int]], k: int) -> List[List[int]]:
+        h = []
+        
+        for x, y in points:
+            dis = (x**2+y**2)**0.5
+            heapq.heappush(h, (-dis, x, y))
+            if len(h)>k: heapq.heappop(h)
+        
+        return [(x, y) for dis, x, y in h]
\ No newline at end of file
diff --git a/problems/python3/koko-eating-bananas.py b/problems/python3/koko-eating-bananas.py
new file mode 100644
index 0000000..fe03197
--- /dev/null
+++ b/problems/python3/koko-eating-bananas.py
@@ -0,0 +1,21 @@
+class Solution:
+    def minEatingSpeed(self, piles: List[int], h: int) -> int:
+        def canFinish(k):
+            timeNeeded = 0
+            for pile in piles:
+                timeNeeded += math.ceil(pile/k)
+            return timeNeeded<=h
+        
+        kMin = 1
+        kMax = max(piles)
+        ans = kMax
+        
+        while kMin<=kMax:
+            k = kMin + int((kMax-kMin)/2)
+            if canFinish(k):
+                ans = min(ans, k)
+                kMax = k-1
+            else:
+                kMin = k+1
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/kth-largest-element-in-a-stream.py b/problems/python3/kth-largest-element-in-a-stream.py
new file mode 100644
index 0000000..1eb9095
--- /dev/null
+++ b/problems/python3/kth-largest-element-in-a-stream.py
@@ -0,0 +1,14 @@
+class KthLargest:
+
+    def __init__(self, k: int, nums: List[int]):
+        self.k = k
+        self.nums = nums
+        
+        heapq.heapify(self.nums)
+        while len(self.nums)>self.k: heapq.heappop(self.nums)
+        
+
+    def add(self, val: int) -> int:
+        heapq.heappush(self.nums, val)
+        if len(self.nums)>self.k: heapq.heappop(self.nums)
+        return self.nums[0]
\ No newline at end of file
diff --git a/problems/python3/kth-smallest-element-in-a-bst.py b/problems/python3/kth-smallest-element-in-a-bst.py
new file mode 100755
index 0000000..c8a1555
--- /dev/null
+++ b/problems/python3/kth-smallest-element-in-a-bst.py
@@ -0,0 +1,21 @@
+"""
+Time: O(N) for the inorder traversal
+Space: O(LogN) if the tree is balanced.
+"""
+class Solution:
+    def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+        count = 0
+        stack = []
+        node = root
+        
+        while stack or node:
+            while node:
+                stack.append(node)
+                node = node.left
+            node = stack.pop()
+            
+            count += 1
+            if count==k: return node.val
+            
+            node = node.right
+        return 0
\ No newline at end of file
diff --git a/problems/python3/largest-rectangle-in-histogram.py b/problems/python3/largest-rectangle-in-histogram.py
new file mode 100644
index 0000000..1cf8508
--- /dev/null
+++ b/problems/python3/largest-rectangle-in-histogram.py
@@ -0,0 +1,21 @@
+"""
+[0] For each height, if the it is lower than the previous one, it means that the previous are not able to extend anymore. So we calculate its area.
+
+[1] If the previous area, is larger than the current one, it means that the current one are able to extand backward.
+"""
+class Solution:
+    def largestRectangleArea(self, heights: List[int]) -> int:
+        maxArea = 0
+        stack = []
+        
+        heights.append(0) #dummy for the ending
+        
+        for i, h in enumerate(heights):
+            start = i
+            while stack and h<stack[-1][1]: #[0]
+                j, h2 = stack.pop()
+                maxArea = max(maxArea, h2*(i-j))
+                start = j
+            stack.append((start, h)) #[1]
+        
+        return maxArea
\ No newline at end of file
diff --git a/problems/python3/last-stone-weight.py b/problems/python3/last-stone-weight.py
new file mode 100644
index 0000000..9b30665
--- /dev/null
+++ b/problems/python3/last-stone-weight.py
@@ -0,0 +1,12 @@
+class Solution:
+    def lastStoneWeight(self, stones: List[int]) -> int:
+        stones = [-stone for stone in stones]
+        heapq.heapify(stones)
+        
+        while len(stones)>=2:
+            w1 = -heapq.heappop(stones)
+            w2 = -heapq.heappop(stones)
+            
+            if w1-w2>0: heapq.heappush(stones, -(w1-w2))
+        
+        return -stones[0] if stones else 0
\ No newline at end of file
diff --git a/problems/python3/letter-combinations-of-a-phone-number.py b/problems/python3/letter-combinations-of-a-phone-number.py
new file mode 100644
index 0000000..582106c
--- /dev/null
+++ b/problems/python3/letter-combinations-of-a-phone-number.py
@@ -0,0 +1,25 @@
+"""
+Time: O(4^N * N), there are around 4^N of combination. Each taking O(N) to form.
+Space: O(N). O(N) for recursion stack. O(N). O(N) for `combination`. O(N+N) ~= O(N).
+"""
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
+        def helper(i):
+            if i==len(digits):
+                ans.append(''.join(combination))
+                return
+            
+            for c in mapping[digits[i]]:
+                combination.append(c)
+                helper(i+1)
+                combination.pop()
+        
+        mapping = {'2': ('a', 'b', 'c'), '3': ('d', 'e', 'f'),
+            '4': ('g', 'h', 'i'), '5': ('j', 'k', 'l'), '6': ('m', 'n', 'o'),
+            '7': ('p', 'q', 'r', 's'), '8': ('t', 'u', 'v'), '9': ('w', 'x', 'y', 'z')}
+        
+        if not digits: return []
+        ans = []
+        combination = []
+        helper(0)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/linked-list-cycle.py b/problems/python3/linked-list-cycle.py
new file mode 100644
index 0000000..d2e5395
--- /dev/null
+++ b/problems/python3/linked-list-cycle.py
@@ -0,0 +1,16 @@
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        if not head: return False
+        
+        slow = head
+        fast = head
+        
+        while fast:
+            slow = slow.next
+            
+            if not fast.next: return False
+            fast = fast.next.next
+            
+            if slow==fast: return True
+        
+        return False
\ No newline at end of file
diff --git a/problems/python3/longest-common-subsequence.py b/problems/python3/longest-common-subsequence.py
new file mode 100644
index 0000000..5ba3a68
--- /dev/null
+++ b/problems/python3/longest-common-subsequence.py
@@ -0,0 +1,16 @@
+class Solution:
+    def longestCommonSubsequence(self, text1: str, text2: str) -> int:
+        #dp[i][j] := number of longest Common Subsequence with text2[:i] and text2[:j]
+        
+        N = len(text1)
+        M = len(text2)
+        
+        dp = [[0]*(M+1) for _ in range(N+1)]
+        
+        for i in range(1, N+1):
+            for j in range(1, M+1):
+                if text1[i-1]==text2[j-1]:
+                    dp[i][j] = dp[i-1][j-1]+1
+                else:
+                    dp[i][j] = max(dp[i][j-1], dp[i-1][j])
+        return dp[-1][-1]
\ No newline at end of file
diff --git a/problems/python3/longest-consecutive-sequence.py b/problems/python3/longest-consecutive-sequence.py
new file mode 100644
index 0000000..dec2cfa
--- /dev/null
+++ b/problems/python3/longest-consecutive-sequence.py
@@ -0,0 +1,15 @@
+class Solution:
+    def longestConsecutive(self, nums: List[int]) -> int:
+        numSet = set(nums)
+        ans = 0
+        
+        for num in nums:
+            isStart = num-1 not in numSet
+            if isStart:
+                count = 0
+                temp = num
+                while temp in numSet:
+                    count += 1
+                    temp += 1
+                ans = max(count, ans)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/longest-increasing-path-in-a-matrix.py b/problems/python3/longest-increasing-path-in-a-matrix.py
new file mode 100644
index 0000000..99fed7d
--- /dev/null
+++ b/problems/python3/longest-increasing-path-in-a-matrix.py
@@ -0,0 +1,26 @@
+"""
+Time: O(MN) since the memo at most has MN index.
+Space: O(MN)
+"""
+class Solution:
+    def longestIncreasingPath(self, matrix: List[List[int]]) -> int:
+        def dfs(i0, j0):
+            if (i0, j0) in memo: return memo[(i0, j0)]
+            ans = 1
+            
+            for i, j in ((i0+1, j0), (i0-1, j0), (i0, j0+1),(i0, j0-1)):
+                if i<0 or i>=N or j<0 or j>=M: continue
+                if matrix[i][j]<=matrix[i0][j0]: continue
+                ans = max(ans, 1+dfs(i, j))
+            
+            memo[(i0, j0)] = ans
+            return ans
+            
+        N = len(matrix)
+        M = len(matrix[0])
+        memo = {}
+        ans = 0
+        for i in range(N):
+            for j in range(M):
+                ans = max(ans, dfs(i, j))
+        return ans
\ No newline at end of file
diff --git a/problems/python3/longest-palindromic-substring.py b/problems/python3/longest-palindromic-substring.py
new file mode 100644
index 0000000..9edb477
--- /dev/null
+++ b/problems/python3/longest-palindromic-substring.py
@@ -0,0 +1,45 @@
+"""
+Time: O(N^2)
+Space: O(N^2)
+
+DP, TLE
+"""
+class Solution:
+    def longestPalindrome(self, s: str) -> str:
+        ans = s[0]
+        N = len(s)
+        dp = [[False]*N for _ in range(N)]
+        
+        for i in range(N): dp[i][i] = True
+        
+        for l in range(2, N+1):
+            for i in range(N):
+                j = i+l-1
+                if j>=N: continue
+                dp[i][j] = s[i]==s[j] and (dp[i+1][j-1] or j-1<i+1)
+                if dp[i][j]: ans = s[i:j+1]
+        return ans
+
+
+"""
+Time: O(N^2)
+Space: O(1)
+"""
+class Solution:
+    def longestPalindrome(self, s: str) -> str:
+        N = len(s)
+        ans = s[0]
+        
+        for i in range(N):
+            l, r = i, i
+            while l>=0 and r<N and s[l]==s[r]:
+                if r-l+1>len(ans): ans = s[l:r+1]
+                l -= 1
+                r += 1
+            
+            l, r = i, i+1
+            while l>=0 and r<N and s[l]==s[r]:
+                if r-l+1>len(ans): ans = s[l:r+1]
+                l -= 1
+                r += 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/longest-repeating-character-replacement.py b/problems/python3/longest-repeating-character-replacement.py
new file mode 100644
index 0000000..714d8f1
--- /dev/null
+++ b/problems/python3/longest-repeating-character-replacement.py
@@ -0,0 +1,13 @@
+class Solution:
+    def characterReplacement(self, s: str, k: int) -> int:
+        counter = collections.Counter()
+        l = 0
+        ans = 0
+        
+        for r in range(len(s)):
+            counter[s[r]] += 1
+            while (r-l+1)-max(counter.values()) > k:
+                counter[s[l]] -= 1
+                l += 1
+            ans = max(ans, r-l+1)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/longest-substring-without-repeating-char.py b/problems/python3/longest-substring-without-repeating-char.py
new file mode 100644
index 0000000..9902bb5
--- /dev/null
+++ b/problems/python3/longest-substring-without-repeating-char.py
@@ -0,0 +1,14 @@
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        ans = 0
+        l = 0
+        seen = set()
+        
+        for r in range(len(s)):
+            while s[r] in seen:
+                seen.remove(s[l])
+                l += 1
+            seen.add(s[r])
+            ans = max(ans, r-l+1)
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/lowest-common-ancestor-of-a-binary-search-tree.py b/problems/python3/lowest-common-ancestor-of-a-binary-search-tree.py
new file mode 100755
index 0000000..12e1743
--- /dev/null
+++ b/problems/python3/lowest-common-ancestor-of-a-binary-search-tree.py
@@ -0,0 +1,28 @@
+"""
+Recursive
+Time: O(LogN) if the tree is balanced.
+Space: O(LogN) for the recursion stack.
+"""
+class Solution:
+    def lowestCommonAncestor(self, node: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
+        if q.val<=node.val<=p.val or p.val<=node.val<=q.val:
+            return node
+        elif q.val<node.val and p.val<node.val:
+            return self.lowestCommonAncestor(node.left, p, q)
+        elif node.val<q.val and node.val<p.val:
+            return self.lowestCommonAncestor(node.right, p, q)
+
+
+"""
+Iterative
+Time: O(LogN) if the tree is balanced.
+Space: O(1).
+"""
+class Solution:
+    def lowestCommonAncestor(self, node: 'TreeNode', p: 'TreeNode', q: 'TreeNode') -> 'TreeNode':
+        while not (q.val<=node.val<=p.val or p.val<=node.val<=q.val):
+            if q.val<node.val and p.val<node.val:
+                node = node.left
+            elif node.val<q.val and node.val<p.val:
+                node = node.right
+        return node
\ No newline at end of file
diff --git a/problems/python3/lru-cache.py b/problems/python3/lru-cache.py
new file mode 100644
index 0000000..7cb1822
--- /dev/null
+++ b/problems/python3/lru-cache.py
@@ -0,0 +1,47 @@
+class Node:
+    def __init__(self, key: int, val: int):
+        self.key = key
+        self.val = val
+        self.next = None
+        self.prev = None
+
+class LRUCache:
+    
+    def __init__(self, capacity: int):
+        self.capacity = capacity
+        self.dic = {} #[0]
+        self.head = Node(0, 0) #[3]
+        self.tail = Node(0, 0) #[3]
+        self.head.next = self.tail
+        self.tail.prev = self.head
+        
+    def remove(self, node):
+        node.prev.next = node.next
+        node.next.prev = node.prev
+    
+    def promote(self, node): #[1]
+        #set the node next to head
+        temp = self.head.next
+        node.next = temp
+        temp.prev = node
+        self.head.next = node
+        node.prev = self.head
+
+    def get(self, key: int) -> int:
+        if key in self.dic:
+            node = self.dic[key]
+            self.remove(node)
+            self.promote(node)
+            return node.val
+        return -1
+            
+    def put(self, key: int, value: int) -> None:
+        if key in self.dic:
+            self.remove(self.dic[key])
+        node = Node(key, value)
+        self.promote(node)
+        self.dic[key] = node
+        
+        if len(self.dic)>self.capacity: #[2]
+            del self.dic[self.tail.prev.key]
+            self.remove(self.tail.prev)
diff --git a/problems/python3/max-area-of-island.py b/problems/python3/max-area-of-island.py
new file mode 100644
index 0000000..fd55a94
--- /dev/null
+++ b/problems/python3/max-area-of-island.py
@@ -0,0 +1,23 @@
+class Solution:
+    def maxAreaOfIsland(self, grid: List[List[int]]) -> int:
+        def dfs(i, j) -> int:
+            if i<0 or j<0 or i>=MAX_ROW or j>=MAX_COL: return 0
+            if grid[i][j]==0 or grid[i][j]==2: return 0
+            
+            grid[i][j] = 2 #mark as visited
+            
+            area = 1
+            area += dfs(i+1, j)
+            area += dfs(i-1, j)
+            area += dfs(i, j+1)
+            area += dfs(i, j-1)
+            return area
+            
+        ans = 0
+        MAX_ROW = len(grid)
+        MAX_COL = len(grid[0])
+        
+        for i in range(MAX_ROW):
+            for j in range(MAX_COL):
+                ans = max(ans, dfs(i, j))
+        return ans
\ No newline at end of file
diff --git a/problems/python3/maximum-depth-of-binary-tree.py b/problems/python3/maximum-depth-of-binary-tree.py
new file mode 100755
index 0000000..3146dcf
--- /dev/null
+++ b/problems/python3/maximum-depth-of-binary-tree.py
@@ -0,0 +1,8 @@
+"""
+Time: O(N)
+Space:O(LogN), for recursion stack space.
+"""
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root: return 0
+        return 1+max(self.maxDepth(root.left), self.maxDepth(root.right))
\ No newline at end of file
diff --git a/problems/python3/maximum-product-subarray.py b/problems/python3/maximum-product-subarray.py
new file mode 100644
index 0000000..243bf41
--- /dev/null
+++ b/problems/python3/maximum-product-subarray.py
@@ -0,0 +1,25 @@
+"""
+Time: O(N)
+Space: O(N), can be reduce to O(1)
+
+dp[i][0] := The max product from subarray that end with nums[i]
+dp[i][1] := The min product from subarray that end with nums[i]
+"""
+class Solution:
+    def maxProduct(self, nums: List[int]) -> int:
+        N = len(nums)
+        dp = [[1, 1] for _ in range(N+1)]
+        ans = float('-inf')
+        
+        for i in range(1, N+1):
+            dp[i][0] = dp[i][1] = nums[i-1]
+            
+            if nums[i-1]>0:
+                dp[i][0] = max(dp[i][0], nums[i-1]*dp[i-1][0])
+                dp[i][1] = min(dp[i][1], nums[i-1]*dp[i-1][1])
+            else:
+                dp[i][0] = max(dp[i][0], nums[i-1]*dp[i-1][1])
+                dp[i][1] = min(dp[i][1], nums[i-1]*dp[i-1][0])
+            ans = max(ans, dp[i][0])
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/maximum-subarray.py b/problems/python3/maximum-subarray.py
new file mode 100644
index 0000000..6f749fa
--- /dev/null
+++ b/problems/python3/maximum-subarray.py
@@ -0,0 +1,17 @@
+"""
+Time: O(N)
+Space: O(1)
+
+Calculate the prefix sum. Whenever the prefix is negative, we ignore all the value before.
+Update the ans along the way.
+"""
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        ans = float('-inf')
+        currSum = 0
+        
+        for num in nums:
+            if currSum<0: currSum = 0
+            currSum += num
+            ans = max(ans, currSum)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/median-of-two-sorted-arrays.py b/problems/python3/median-of-two-sorted-arrays.py
new file mode 100644
index 0000000..0fa31da
--- /dev/null
+++ b/problems/python3/median-of-two-sorted-arrays.py
@@ -0,0 +1,40 @@
+"""
+Try to find a i on array A and corespoding j on array B.
+
+Aleft = A[i]
+Aright = A[i+1]
+Bleft = B[j]
+Bright = B[j+1]
+
+Such that Aleft<=Bright and Bleft<=Aright
+
+This means that A[:i+1] and B[:j+1] holds all the elements that is less than the median.
+Aright A[i+1:] and B[j+1:] holds all the elements that is larger or equal to the median.
+"""
+class Solution:
+    def findMedianSortedArrays(self, A: List[int], B: List[int]) -> float:
+        if len(A)>len(B): A, B = B, A
+        
+        total = len(A)+len(B)
+        half = total//2
+        l = 0
+        r = len(A)-1
+        
+        while True:
+            i = l + (r-l)//2
+            j = half-(i+1)-1
+            
+            Aleft = A[i] if i>=0 else float('-inf')
+            Aright = A[i+1] if i+1<len(A) else float('inf')
+            Bleft = B[j] if j>=0 else float('-inf')
+            Bright = B[j+1] if j+1<len(B) else float('inf')
+            
+            if Aleft<=Bright and Bleft<=Aright:
+                if total%2==0:
+                    return (max(Aleft, Bleft)+min(Aright, Bright))/2
+                else:
+                    return min(Aright, Bright)
+            elif Aleft>Bright:
+                r = i-1
+            else:
+                l = i+1
\ No newline at end of file
diff --git a/problems/python3/meeting-rooms-ii.py b/problems/python3/meeting-rooms-ii.py
new file mode 100644
index 0000000..1eb4eb4
--- /dev/null
+++ b/problems/python3/meeting-rooms-ii.py
@@ -0,0 +1,12 @@
+class Solution:
+    def minMeetingRooms(self, intervals: List[List[int]]) -> int:
+        intervals.sort()
+        h = []
+        ans = 0
+        
+        for s, e in intervals:
+            while h and s>=h[0][0]:
+                heapq.heappop(h)
+            heapq.heappush(h, (e, s))
+            ans = max(ans, len(h))
+        return ans
\ No newline at end of file
diff --git a/problems/python3/meeting-rooms.py b/problems/python3/meeting-rooms.py
new file mode 100644
index 0000000..60cf429
--- /dev/null
+++ b/problems/python3/meeting-rooms.py
@@ -0,0 +1,10 @@
+class Solution:
+    def canAttendMeetings(self, intervals: List[List[int]]) -> bool:
+        intervals.sort()
+        lastEnd = float('-inf')
+        for s, e in intervals:
+            if lastEnd>s:
+                return False
+            else:
+                lastEnd = e
+        return True
\ No newline at end of file
diff --git a/problems/python3/merge-intervals.py b/problems/python3/merge-intervals.py
new file mode 100644
index 0000000..c8a4678
--- /dev/null
+++ b/problems/python3/merge-intervals.py
@@ -0,0 +1,17 @@
+"""
+Time: O(NLogN) for sorting
+Space: O(1) excluding the output.
+"""
+class Solution:
+    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
+        ans = []
+        intervals.sort()
+        
+        for s, e in intervals:
+            if not ans:
+                ans.append([s, e])
+            elif ans[-1][1]>=s:
+                ans[-1][1] = max(ans[-1][1], e)
+            else:
+                ans.append([s, e])
+        return ans
\ No newline at end of file
diff --git a/problems/python3/merge-triplets-to-form-target-triplet.py b/problems/python3/merge-triplets-to-form-target-triplet.py
new file mode 100644
index 0000000..4708e4f
--- /dev/null
+++ b/problems/python3/merge-triplets-to-form-target-triplet.py
@@ -0,0 +1,15 @@
+"""
+If any element in the triplets is larger than the element in target, it cannot be used.
+Check if we have all 3 index found the same value.
+"""
+class Solution:
+    def mergeTriplets(self, triplets: List[List[int]], target: List[int]) -> bool:
+        okIndex = set()
+        
+        for a, b, c in triplets:
+            if a>target[0] or b>target[1] or c>target[2]: continue
+            if a==target[0]: okIndex.add(0)
+            if b==target[1]: okIndex.add(1)
+            if c==target[2]: okIndex.add(2)
+        
+        return len(okIndex)==3
\ No newline at end of file
diff --git a/problems/python3/merge-two-sorted-lists.py b/problems/python3/merge-two-sorted-lists.py
new file mode 100644
index 0000000..2b31352
--- /dev/null
+++ b/problems/python3/merge-two-sorted-lists.py
@@ -0,0 +1,15 @@
+class Solution:
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        head = ListNode() #dummy
+        node = head
+        while list1 and list2:
+            if list1.val<list2.val:
+                node.next = list1
+                list1 = list1.next
+            else:
+                node.next = list2
+                list2 = list2.next
+            node = node.next
+        
+        node.next = list1 or list2
+        return head.next
\ No newline at end of file
diff --git a/problems/python3/min-cost-climbing-stairs.py b/problems/python3/min-cost-climbing-stairs.py
new file mode 100644
index 0000000..c59c446
--- /dev/null
+++ b/problems/python3/min-cost-climbing-stairs.py
@@ -0,0 +1,16 @@
+"""
+Time: O(N)
+Space: O(N), can further reduce to using only 2 variables -> O(1).
+
+dp[i] := the cost to get to index i.
+"""
+class Solution:
+    def minCostClimbingStairs(self, cost: List[int]) -> int:
+        N = len(cost)
+        dp = [float('inf')]*(N+1)
+        dp[0] = 0
+        dp[1] = 0
+        
+        for i in range(2, len(dp)):
+            dp[i] = min(dp[i-1]+cost[i-1], dp[i-2]+cost[i-2])
+        return dp[-1]
\ No newline at end of file
diff --git a/problems/python3/min-cost-to-connect-all-points.py b/problems/python3/min-cost-to-connect-all-points.py
new file mode 100644
index 0000000..4cd6ada
--- /dev/null
+++ b/problems/python3/min-cost-to-connect-all-points.py
@@ -0,0 +1,28 @@
+class Solution:
+    def minCostConnectPoints(self, points: List[List[int]]) -> int:
+        ans = 0
+        visited = set()
+        adj = collections.defaultdict(list)
+        N = len(points)
+        
+        #build adjacency list
+        for i in range(N):
+            x0, y0 = points[i]
+            for j in range(i+1, N):
+                x1, y1 = points[j]
+                dis = abs(x0-x1)+abs(y0-y1)
+                adj[(x0, y0)].append((dis, x1, y1))
+                adj[(x1, y1)].append((dis, x0, y0))
+        
+        h = [(0, points[0][0], points[0][1])] #min heap
+        while len(visited)<N:
+            dis, x, y = heapq.heappop(h)
+            
+            if (x, y) in visited: continue
+            visited.add((x, y))
+            ans += dis
+            
+            for dis1, x1, y1 in adj[(x, y)]:
+                if (x1, y1) in visited: continue
+                heapq.heappush(h, (dis1, x1, y1))
+        return ans
\ No newline at end of file
diff --git a/problems/python3/min-stack.py b/problems/python3/min-stack.py
new file mode 100644
index 0000000..8fe03de
--- /dev/null
+++ b/problems/python3/min-stack.py
@@ -0,0 +1,23 @@
+#"stack" is just a normal stack.
+#"minStack" stores the min of the stack element in the same position.
+#i.e. At the time stack[x] put in to the stack. minStack[x] is the min.
+
+class MinStack:
+
+    def __init__(self):
+        self.stack = []
+        self.minStack = []
+
+    def push(self, val: int) -> None:
+        self.stack.append(val)
+        self.minStack.append(val if (not self.minStack or val<self.minStack[-1]) else self.minStack[-1])
+
+    def pop(self) -> None:
+        self.minStack.pop()
+        return self.stack.pop()
+        
+    def top(self) -> int:
+        return self.stack[-1]
+        
+    def getMin(self) -> int:
+        return self.minStack[-1]
\ No newline at end of file
diff --git a/problems/python3/minimum-interval-to-include-each-query.py b/problems/python3/minimum-interval-to-include-each-query.py
new file mode 100644
index 0000000..ca43de3
--- /dev/null
+++ b/problems/python3/minimum-interval-to-include-each-query.py
@@ -0,0 +1,23 @@
+class Solution:
+    def minInterval(self, intervals: List[List[int]], queries: List[int]) -> List[int]:
+        ans = [-1]*len(queries)
+        queries = sorted([(query, i) for i, query in enumerate(queries)])
+        intervals.sort()
+        h = []
+        
+        itervalIndex = 0
+        for query, queryIndex in queries:
+            #push all intervals that include 'query' to the min heap
+            while itervalIndex<len(intervals) and intervals[itervalIndex][0]<=query:
+                left = intervals[itervalIndex][0]
+                right = intervals[itervalIndex][1]
+                size = right-left+1
+                heapq.heappush(h, (size, right))
+                itervalIndex += 1
+            
+            #pop all intervals that not includes 'query' out of the min heap
+            while h and h[0][1]<query:
+                heapq.heappop(h)
+                
+            if h: ans[queryIndex] = h[0][0]
+        return ans
\ No newline at end of file
diff --git a/problems/python3/minimum-window-substring.py b/problems/python3/minimum-window-substring.py
new file mode 100644
index 0000000..1a23a82
--- /dev/null
+++ b/problems/python3/minimum-window-substring.py
@@ -0,0 +1,25 @@
+class Solution:
+    def minWindow(self, s: str, t: str) -> str:
+        if len(t)>len(s): return ""
+        
+        ans = ""
+        counter1 = collections.Counter(t)
+        charSet = set(t)
+        counter2 = collections.Counter() #sliding window in string s, index between l and r
+        charSet2 = set(s)
+        matchCount = 0 #count of char in the sliding window that counts are larger than the char count in t.
+        
+        l = 0
+        for r in range(len(s)):
+            counter2[s[r]] += 1
+            
+            if s[r] in charSet and counter1[s[r]]==counter2[s[r]]: matchCount += 1
+                
+            while l<len(s) and (s[l] not in charSet or counter2[s[l]]>counter1[s[l]]):
+                counter2[s[l]] -= 1
+                l += 1
+                
+            if matchCount==len(charSet) and (ans=="" or r-l+1<len(ans)):
+                ans = s[l:r+1]
+                
+        return ans
\ No newline at end of file
diff --git a/problems/python3/missing-number.py b/problems/python3/missing-number.py
new file mode 100644
index 0000000..f0531f6
--- /dev/null
+++ b/problems/python3/missing-number.py
@@ -0,0 +1,12 @@
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        N = len(nums)
+        ans = 0
+        
+        for n in range(N+1):
+            ans ^= n
+        
+        for n in nums:
+            ans ^= n
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/multiply-strings.py b/problems/python3/multiply-strings.py
new file mode 100644
index 0000000..be9f675
--- /dev/null
+++ b/problems/python3/multiply-strings.py
@@ -0,0 +1,22 @@
+class Solution:
+    def multiply(self, num1: str, num2: str) -> str:
+        if num1=='0' or num2=='0': return '0'
+        M, N = len(num1), len(num2)
+        temp = [0]*(M+N+1)
+
+        num1, num2 = num1[::-1], num2[::-1]
+        for i in range(M):
+            for j in range(N):
+                digits = int(num1[i])*int(num2[j])
+                temp[i+j] += digits
+                temp[i+j+1] += temp[i+j]//10
+                temp[i+j] = temp[i+j]%10
+        
+        ans = ''
+        temp = temp[::-1]
+        isLeadingZero = True
+        for d in temp:
+            if d!=0 or not isLeadingZero:
+                isLeadingZero = False
+                ans += str(d)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/n-queens.py b/problems/python3/n-queens.py
new file mode 100644
index 0000000..2ab7335
--- /dev/null
+++ b/problems/python3/n-queens.py
@@ -0,0 +1,45 @@
+class Solution:
+    def solveNQueens(self, n: int) -> List[List[str]]:
+        def helper(row: int):
+            if row==n:
+                ans.append(convertFormat(queenCols))
+                return
+            
+            for col in range(n):
+                posDiag = row+col
+                negDiag = row-col
+                
+                if col in colUsed or posDiag in posDiagUsed or negDiag in negDiagUsed: continue
+                
+                queenCols.append(col)
+                colUsed.add(col)
+                posDiagUsed.add(posDiag)
+                negDiagUsed.add(negDiag)
+                
+                helper(row+1)
+                
+                queenCols.pop()
+                colUsed.remove(col)
+                posDiagUsed.remove(posDiag)
+                negDiagUsed.remove(negDiag)
+        
+        def convertFormat(queenCols: List[int]) -> List[str]:
+            output = []
+            for col in queenCols:
+                row = ''
+                for i in range(n):
+                    if i==col:
+                        row += 'Q'
+                    else:
+                        row += '.'
+                output.append(row)
+            return output
+            
+        ans = []
+        queenCols = []
+        colUsed = set()
+        posDiagUsed = set()
+        negDiagUsed = set()
+        
+        helper(0)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/network-delay-time.py b/problems/python3/network-delay-time.py
new file mode 100644
index 0000000..6da8613
--- /dev/null
+++ b/problems/python3/network-delay-time.py
@@ -0,0 +1,23 @@
+class Solution:
+    def networkDelayTime(self, times: List[List[int]], n: int, k: int) -> int:
+        ans = 0
+        adj = collections.defaultdict(list)
+        h = []
+        visited = set()
+        
+        for u, v, w in times:
+            adj[u].append((v, w))
+            
+        heapq.heappush(h, (0, k))
+        while h:
+            timeNeededToGetHere, node = heapq.heappop(h)
+            
+            if node in visited: continue
+            visited.add(node)
+            ans = max(ans, timeNeededToGetHere)
+            
+            for nei, time in adj[node]:
+                if nei in visited: continue
+                heapq.heappush(h, (time+timeNeededToGetHere, nei))
+        
+        return ans if len(visited)==n else -1
\ No newline at end of file
diff --git a/problems/python3/non-overlapping-intervals.py b/problems/python3/non-overlapping-intervals.py
new file mode 100644
index 0000000..b3b6a0d
--- /dev/null
+++ b/problems/python3/non-overlapping-intervals.py
@@ -0,0 +1,17 @@
+"""
+Time: O(NLogN) for sorting.
+"""
+class Solution:
+    def eraseOverlapIntervals(self, intervals: List[List[int]]) -> int:
+        intervals.sort()
+        
+        ans = 0
+        prevEnd = intervals[0][1]
+        
+        for s, e in intervals[1:]:
+            if s>=prevEnd:
+                prevEnd = e
+            else:
+                ans += 1
+                prevEnd = min(prevEnd, e)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/number-of-1-bits.py b/problems/python3/number-of-1-bits.py
new file mode 100644
index 0000000..2273859
--- /dev/null
+++ b/problems/python3/number-of-1-bits.py
@@ -0,0 +1,10 @@
+"""
+n = n&(n-1) will turn the right most 1 to 0.
+"""
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        ans = 0
+        while n>0:
+            n = n&(n-1)
+            ans += 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/number-of-connected-components-in-an-undirected-graph.py b/problems/python3/number-of-connected-components-in-an-undirected-graph.py
new file mode 100644
index 0000000..28571ea
--- /dev/null
+++ b/problems/python3/number-of-connected-components-in-an-undirected-graph.py
@@ -0,0 +1,25 @@
+class Solution:
+    def countComponents(self, N: int, edges: List[List[int]]) -> int:
+        def union(n1, n2):
+            p1 = find(n1)
+            p2 = find(n2)
+            if p1==p2:
+                return
+            elif p1<p2:
+                parents[p2] = p1
+            else:
+                parents[p1] = p2
+            
+        def find(n):
+            p = parents[n]
+            while p!=parents[p]: p = find(p)
+            parents[n] = p
+            return p
+        
+        parents = {n:n for n in range(N)}
+        for n1, n2 in edges: union(n1, n2)
+        
+        groups = set()
+        for n in range(N): groups.add(find(n))
+            
+        return len(groups)
\ No newline at end of file
diff --git a/problems/python3/number-of-islands.py b/problems/python3/number-of-islands.py
new file mode 100644
index 0000000..6cc325e
--- /dev/null
+++ b/problems/python3/number-of-islands.py
@@ -0,0 +1,22 @@
+class Solution:
+    def numIslands(self, grid: List[List[str]]) -> int:
+        def dfs(i, j):
+            if i<0 or j<0 or i>=MAX_ROWS or j>=MAX_COLS: return
+            if grid[i][j]!='1': return
+            
+            grid[i][j] = '2'
+            dfs(i+1, j)
+            dfs(i-1, j)
+            dfs(i, j+1)
+            dfs(i, j-1)
+        
+        count = 0
+        MAX_ROWS = len(grid)
+        MAX_COLS = len(grid[0])
+        
+        for i in range(MAX_ROWS):
+            for j in range(MAX_COLS):
+                if grid[i][j]=='1':
+                    dfs(i, j)
+                    count += 1
+        return count
\ No newline at end of file
diff --git a/problems/python3/pacific-atlantic-water-flow.py b/problems/python3/pacific-atlantic-water-flow.py
new file mode 100644
index 0000000..05032b0
--- /dev/null
+++ b/problems/python3/pacific-atlantic-water-flow.py
@@ -0,0 +1,30 @@
+class Solution:
+    def pacificAtlantic(self, heights: List[List[int]]) -> List[List[int]]:
+        def bfs(q, ocian):
+            while q:
+                i0, j0 = q.popleft()
+                if (i0, j0) in ocian: continue
+                ocian.add((i0, j0))
+                
+                for i, j in ((i0+1, j0), (i0-1, j0), (i0, j0+1), (i0, j0-1)):
+                    if i<0 or j<0 or i>=MAX_ROW or j>=MAX_COL: continue
+                    if heights[i][j]>=heights[i0][j0]: q.append((i, j))
+                    
+        MAX_ROW = len(heights)
+        MAX_COL = len(heights[0])
+        
+        #add the top and left to q1
+        pacific = set()
+        q1 = collections.deque()
+        for j in range(MAX_COL): q1.append((0, j))
+        for i in range(MAX_ROW): q1.append((i, 0))
+            
+        #add botton and right to q2
+        atlantic = set()
+        q2 = collections.deque()
+        for j in range(MAX_COL): q2.append((MAX_ROW-1, j))
+        for i in range(MAX_ROW): q2.append((i, MAX_COL-1))
+            
+        bfs(q1, pacific)
+        bfs(q2, atlantic)
+        return pacific.intersection(atlantic)
\ No newline at end of file
diff --git a/problems/python3/palindrome-partitioning.py b/problems/python3/palindrome-partitioning.py
new file mode 100644
index 0000000..d02ef17
--- /dev/null
+++ b/problems/python3/palindrome-partitioning.py
@@ -0,0 +1,64 @@
+"""
+`helper(i)` finds all the palindrome substring from i to j see if we can get an answer by recursively calling `helper(j+1)`.
+
+Time: O(2^N * N), for a string length S, there will be 2^N combination of substrings. For each substring, it will take O(N) to test if they are all palindrome.
+Space: O(N). `partition` will takes O(N) and recursion stack will also take O(N). O(N + N) ~= O(N)
+"""
+class Solution:
+    def partition(self, s: str) -> List[List[str]]:
+        def helper(i):
+            if i>=len(s):
+                ans.append(partition.copy())
+                return
+            
+            for j in range(i, len(s)):
+                if isPalindrome(i, j):
+                    partition.append(s[i:j+1])
+                    helper(j+1)
+                    partition.pop()
+
+        def isPalindrome(i, j):
+            while i<=j:
+                if s[i]!=s[j]: return False
+                i += 1
+                j -= 1
+            return True
+                
+        ans = []
+        partition = []
+        helper(0)
+        return ans
+
+"""
+If you look closely you can see that we execute `isPalindrome` on many repeated substrings.
+We can optimize this by storing the result in `dp` and reuse it.
+Since all the less difference i and j (shorter substring s[i:j+1]) will be process first in the `isPalindrome`.
+When checking if s[i:j+1] is a palindrome or not, we can simply check the if s[i]==s[j] and the previous result (dp[i+1][j-1])
+
+Time: O(2^N * N), The overall time complexity is the same, but isPalindrome is actually a lot faster.
+Space: O(N^2) for `dp`.
+"""
+class Solution:
+    def partition(self, s: str) -> List[List[str]]:
+        def helper(i):
+            if i>=N:
+                ans.append(partition.copy())
+                return
+            
+            for j in range(i, N):
+                if isPalindrome(i, j):
+                    partition.append(s[i:j+1])
+                    helper(j+1)
+                    partition.pop()
+
+        def isPalindrome(i, j):
+            dp[i][j] = i==j or (j-i==1 and s[i]==s[j]) or (s[i]==s[j] and dp[i+1][j-1]) #len==1 palindrome or len==2 palindrome or len>=3 palindrome
+            return dp[i][j]
+        
+        N = len(s)
+        ans = []
+        partition = []
+        dp = [[False]*N for _ in range(N)]
+
+        helper(0)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/palindromic-substrings.py b/problems/python3/palindromic-substrings.py
new file mode 100644
index 0000000..7ef7e96
--- /dev/null
+++ b/problems/python3/palindromic-substrings.py
@@ -0,0 +1,16 @@
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        def countPalindrome(l, r) -> int:
+            count = 0
+            while l>=0 and r<N and s[l]==s[r]:
+                count += 1
+                l -= 1
+                r += 1
+            return count
+        
+        N = len(s)
+        ans = 0
+        for i in range(N):
+            ans += countPalindrome(i, i)
+            ans += countPalindrome(i, i+1)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/partition-equal-subset-sum.py b/problems/python3/partition-equal-subset-sum.py
new file mode 100644
index 0000000..ca28510
--- /dev/null
+++ b/problems/python3/partition-equal-subset-sum.py
@@ -0,0 +1,20 @@
+"""
+Time: O(2^N), 2^0 + 2^1 + 2^2 + 2^3 + ... 2^(N-1) == 2^N-1
+Space: O(T), T is the sum of nums
+"""
+class Solution:
+    def canPartition(self, nums: List[int]) -> bool:
+        total = sum(nums)
+        if total%2!=0: return False
+        
+        target = total/2
+        possibleSum = set()
+        possibleSum.add(0)
+        for num in nums:
+            temp = set()
+            for p in possibleSum:
+                if p==target or p+num==target: return True
+                temp.add(p)
+                temp.add(p+num)
+            possibleSum = temp
+        return False
\ No newline at end of file
diff --git a/problems/python3/partition-labels.py b/problems/python3/partition-labels.py
new file mode 100644
index 0000000..0c12d00
--- /dev/null
+++ b/problems/python3/partition-labels.py
@@ -0,0 +1,26 @@
+"""
+Time: O(N)
+Space: O(N)
+
+For each char, store its max index in maxIndex.
+Iterate through the string, update the "currMax" along the way.
+currMax is the place we can partition unless it get updated again.
+If the current index is the currMax, update "ans".
+"""
+class Solution:
+    def partitionLabels(self, s: str) -> List[int]:
+        ans = []
+        maxIndex = {}
+        
+        for i, c in enumerate(s):
+            maxIndex[c] = i
+        
+        currMax = 0
+        processedLength = 0
+        for i, c in enumerate(s):
+            currMax = max(currMax, maxIndex[c])
+            if i==currMax:
+                ans.append(i+1 - processedLength)
+                processedLength += ans[-1]
+
+        return ans
\ No newline at end of file
diff --git a/problems/python3/permutation-in-string.py b/problems/python3/permutation-in-string.py
new file mode 100644
index 0000000..cd22781
--- /dev/null
+++ b/problems/python3/permutation-in-string.py
@@ -0,0 +1,28 @@
+class Solution:
+    def checkInclusion(self, s1: str, s2: str) -> bool:
+        if len(s1)>len(s2): return False
+        
+        counter1 = collections.Counter(s1)
+        counter2 = collections.Counter(s2[:len(s1)])
+        matches = 0
+        for c in 'abcdefghijklmnopqrstuvwxyz':
+            if counter1[c]==counter2[c]: matches += 1
+        if matches==26: return True
+        
+        l = 0
+        for r in range(len(s1), len(s2)):
+            counter2[s2[r]] += 1
+            if counter1[s2[r]]==counter2[s2[r]]:
+                matches += 1
+            elif counter1[s2[r]]+1==counter2[s2[r]]:
+                matches -= 1
+            
+            counter2[s2[l]] -= 1
+            if counter1[s2[l]]==counter2[s2[l]]:
+                matches += 1
+            elif counter1[s2[l]]-1==counter2[s2[l]]:
+                matches -= 1
+            l += 1
+            
+            if matches==26: return True
+        return False
\ No newline at end of file
diff --git a/problems/python3/permutations.py b/problems/python3/permutations.py
new file mode 100644
index 0000000..98d536d
--- /dev/null
+++ b/problems/python3/permutations.py
@@ -0,0 +1,28 @@
+"""
+Time: O(N!), since we call helper() N! times.
+Space: O(N) for recursion stacks.
+
+Whenever we call `helper()`, we pick a num that is not "used" and add it to the `permutation`.
+Recursively call the `helper()` until we filled the `permutation`.
+Resotre `permutation` and `used` and try another `num`.
+"""
+class Solution:
+    def permute(self, nums: List[int]) -> List[List[int]]:
+        def helper():
+            if len(permutation)==len(nums):
+                ans.append(permutation.copy())
+                return
+            
+            for num in nums:
+                if num in used: continue
+                used.add(num)
+                permutation.append(num)
+                helper()
+                used.remove(num)
+                permutation.pop()
+        
+        ans = []
+        permutation = []
+        used = set()
+        helper()
+        return ans
\ No newline at end of file
diff --git a/problems/python3/plus-one.py b/problems/python3/plus-one.py
new file mode 100644
index 0000000..8cfedb4
--- /dev/null
+++ b/problems/python3/plus-one.py
@@ -0,0 +1,16 @@
+class Solution:
+    def plusOne(self, digits: List[int]) -> List[int]:
+        i = len(digits)-1
+        needAdditionDigit = True
+
+        while i>=0 and needAdditionDigit:
+            if digits[i]==9:
+                digits[i] = 0
+                i -= 1
+                needAdditionDigit = True
+            else:
+                digits[i] += 1
+                needAdditionDigit = False
+        if needAdditionDigit: digits.insert(0, 1)
+        return digits
+        
\ No newline at end of file
diff --git a/problems/python3/powx-n.py b/problems/python3/powx-n.py
new file mode 100644
index 0000000..2b1b4fc
--- /dev/null
+++ b/problems/python3/powx-n.py
@@ -0,0 +1,14 @@
+class Solution:
+    def myPow(self, x: float, k: int) -> float:
+        if k<0: return 1/self.myPow(x, -k)
+
+        if k==0:
+            return 1
+        elif k==1:
+            return x
+        elif k%2==0:
+            half = self.myPow(x, k//2)
+            return half * half
+        else:
+            half = self.myPow(x, (k-1)//2) 
+            return half * half * x
\ No newline at end of file
diff --git a/problems/python3/product-of-array-except-self.py b/problems/python3/product-of-array-except-self.py
new file mode 100644
index 0000000..2309896
--- /dev/null
+++ b/problems/python3/product-of-array-except-self.py
@@ -0,0 +1,45 @@
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        #left[i] := product of all nums left of nums[i] (not include nums[i])
+        left = [1]
+        temp = 1
+        for num in nums:
+            temp *= num
+            left.append(temp)
+        
+        #right[i] := product of all nums right of nums[i] (not include nums[i])
+        right = []
+        temp = 1
+        for num in reversed(nums):
+            temp *= num
+            right.append(temp)
+        right.reverse()
+        right.append(1)
+        right = right[1:]
+        
+        ans = []
+        for i in range(len(nums)):
+            ans.append(left[i]*right[i])
+        return ans
+
+#In Place
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        N = len(nums)
+        ans = [0]*N
+        
+        #generate "left"
+        ans[0] = 1
+        for i in range(1, N):
+            ans[i] = ans[i-1]*nums[i-1]
+        
+        #generate "right"
+        temp = 1
+        for i in range(N-2, -1, -1):
+            temp *= nums[i+1]
+            ans[i] *= temp
+        
+        return ans
+            
+    
+    
\ No newline at end of file
diff --git a/problems/python3/reconstruct-itinerary.py b/problems/python3/reconstruct-itinerary.py
new file mode 100644
index 0000000..14c928f
--- /dev/null
+++ b/problems/python3/reconstruct-itinerary.py
@@ -0,0 +1,27 @@
+"""
+DFS with backtracking.
+"""
+class Solution:
+    def findItinerary(self, tickets: List[List[str]]) -> List[str]:
+        def dfs(start) -> bool:
+            if len(ans)==len(tickets)+1: return True
+            if start not in adj: return False
+            
+            temp = list(adj[start])
+            for i, arr in enumerate(temp):
+                adj[start].pop(i)
+                ans.append(arr)
+                if dfs(arr): return True
+                adj[start].insert(i, arr)
+                ans.pop()
+            return False
+        
+        ans = ['JFK']
+        adj = collections.defaultdict(list)
+        
+        tickets.sort()
+        for des, arr in tickets:
+            adj[des].append(arr)
+        
+        dfs('JFK')
+        return ans
\ No newline at end of file
diff --git a/problems/python3/redundant-connection.py b/problems/python3/redundant-connection.py
new file mode 100644
index 0000000..47bda2e
--- /dev/null
+++ b/problems/python3/redundant-connection.py
@@ -0,0 +1,32 @@
+class Solution:
+    def findRedundantConnection(self, edges: List[List[int]]) -> List[int]:
+        def union(n1, n2):
+            p1 = find(n1)
+            p2 = find(n2)
+            
+            if p1==p2:
+                return False #union failed, already united.
+            elif p1<p2:
+                roots[p2] = p1
+            else:
+                roots[p1] = p2
+        
+            return True #union success
+        
+        def find(n):
+            if n not in roots:
+                roots[n] = n
+                return n
+            
+            p = roots[n]
+            while p!=roots[p]:
+                p = find(p)
+            roots[n] = p
+            return p
+        
+        roots = {}
+        
+        for n1, n2 in edges:
+            if not union(n1, n2): return (n1, n2)
+        
+        return 'ERROR'
\ No newline at end of file
diff --git a/problems/python3/regular-expression-matching.py b/problems/python3/regular-expression-matching.py
new file mode 100644
index 0000000..93b44e6
--- /dev/null
+++ b/problems/python3/regular-expression-matching.py
@@ -0,0 +1,24 @@
+class Solution:
+    def isMatch(self, s: str, p: str) -> bool:
+        def dfs(i, j):
+            if (i, j) in cache: return cache[(i, j)]
+            if i>=M and j>=N: return True
+            if j>=N: return False
+            
+            match = i<M and (s[i]==p[j] or p[j]=='.')
+            
+            if j+1<N and p[j+1]=='*':
+                cache[(i, j)] = (match and dfs(i+1, j)) or dfs(i, j+2) #(use one or more p[j]) or (use zero p[j])
+                return cache[(i, j)]
+            
+            if match:
+                cache[(i, j)] = dfs(i+1, j+1)
+                return cache[(i, j)]
+            
+            cache[(i, j)] = False
+            return cache[(i, j)]
+        
+        cache = {}
+        M = len(s)
+        N = len(p)
+        return dfs(0, 0)
\ No newline at end of file
diff --git a/problems/python3/remove-nth-node-from-end-of-list.py b/problems/python3/remove-nth-node-from-end-of-list.py
new file mode 100644
index 0000000..b59d7ab
--- /dev/null
+++ b/problems/python3/remove-nth-node-from-end-of-list.py
@@ -0,0 +1,47 @@
+#Two pass.
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        #count the length of the linked list
+        node = head
+        count = 0
+        while node:
+            count += 1
+            node = node.next
+        
+
+        node = head
+        steps = count-n-1
+        
+        #steps==-1 means that we need to remove the first node
+        if steps==-1: return head.next
+        
+        #traverse to the node before the node we wanted to remove
+        while steps>0:
+            node = node.next
+            steps -= 1
+        
+        #remove "node.next"
+        node.next = node.next.next
+        
+        return head
+
+
+#One pass. Fast pointer is ahead of slow pointer by n+1
+#So slow pointer will stop at the node before the node we wanted to remove
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        dummy = ListNode()
+        dummy.next = head
+
+        ahead = n+1
+        fast = dummy
+        slow = dummy
+        
+        while fast:
+            fast = fast.next
+            ahead -= 1
+            if ahead<0: slow = slow.next
+        
+        slow.next = slow.next.next
+
+        return dummy.next
\ No newline at end of file
diff --git a/problems/python3/reorder-list.py b/problems/python3/reorder-list.py
new file mode 100644
index 0000000..89c6ba2
--- /dev/null
+++ b/problems/python3/reorder-list.py
@@ -0,0 +1,37 @@
+class Solution:
+    def reorderList(self, head: Optional[ListNode]) -> None:
+        #find the middle point
+        slow = head
+        fast = head
+        while fast and fast.next:
+            slow = slow.next
+            fast = fast.next.next
+        middle = slow.next
+
+        #reverse the linked list after the middle point
+        middle = self.reverseList(middle)
+
+        #separate the linked list before the middle
+        slow.next = None
+        
+        #merge two linked list
+        node = head
+        while middle and node:
+            nextNode = node.next
+            node.next = middle
+            middle = middle.next
+            node.next.next = nextNode
+            node = nextNode
+        return head
+    
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        pre = None
+        node = head
+        
+        while node:
+            nextNode = node.next
+            node.next = pre
+            if not nextNode: return node
+            pre = node
+            node = nextNode
+        
\ No newline at end of file
diff --git a/problems/python3/reverse-bits.py b/problems/python3/reverse-bits.py
new file mode 100644
index 0000000..dfc5191
--- /dev/null
+++ b/problems/python3/reverse-bits.py
@@ -0,0 +1,7 @@
+class Solution:
+    def reverseBits(self, n: int) -> int:
+        res = 0
+        for i in range(32):
+            bit = (n >> i) & 1
+            res = res | (bit << (31 - i))
+        return res
\ No newline at end of file
diff --git a/problems/python3/reverse-integer.py b/problems/python3/reverse-integer.py
new file mode 100644
index 0000000..76137ab
--- /dev/null
+++ b/problems/python3/reverse-integer.py
@@ -0,0 +1,16 @@
+class Solution:
+    def reverse(self, x: int) -> int:
+        MAX = 2**31-1
+        MIN = -2**31
+        ans = 0
+        
+        while x:
+            digit = int(math.fmod(x, 10))
+            x = int(x/10)
+            
+            if ans>MAX//10 or (ans==MAX//10 and digit>MAX%10): return 0
+            if ans<MIN//10 or (ans==MIN//10 and digit<MIN%10): return 0
+            
+            ans = ans*10 + digit
+            
+        return ans
\ No newline at end of file
diff --git a/problems/python3/reverse-linked-list.py b/problems/python3/reverse-linked-list.py
new file mode 100644
index 0000000..c9c0655
--- /dev/null
+++ b/problems/python3/reverse-linked-list.py
@@ -0,0 +1,21 @@
+#Recursive
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head or not head.next: return head
+        temp = self.reverseList(head.next)
+        head.next.next = head
+        head.next = None
+        return temp
+
+#Iterative
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        pre = None
+        node = head
+        
+        while node:
+            nextNode = node.next
+            node.next = pre
+            if not nextNode: return node
+            pre = node
+            node = nextNode
\ No newline at end of file
diff --git a/problems/python3/rotate-image.py b/problems/python3/rotate-image.py
new file mode 100644
index 0000000..c8a872a
--- /dev/null
+++ b/problems/python3/rotate-image.py
@@ -0,0 +1,16 @@
+class Solution:
+    def rotate(self, matrix: List[List[int]]) -> None:
+        l, r = 0, len(matrix[0])-1
+        
+        while l<r:
+            top = l
+            bottom = r
+            for i in range(r-l):
+                topLeft = matrix[top][l+i]
+                matrix[top][l+i] = matrix[bottom-i][l]
+                matrix[bottom-i][l] = matrix[bottom][r-i]
+                matrix[bottom][r-i] = matrix[top+i][r]
+                matrix[top+i][r] = topLeft
+            l += 1
+            r -= 1
+            #the outer layer is finisthed, going into the inner layer
\ No newline at end of file
diff --git a/problems/python3/rotting-oranges.py b/problems/python3/rotting-oranges.py
new file mode 100644
index 0000000..2e1216a
--- /dev/null
+++ b/problems/python3/rotting-oranges.py
@@ -0,0 +1,35 @@
+class Solution:
+    def orangesRotting(self, grid: List[List[int]]) -> int:
+        time = 0
+        rotten = set()
+        aboutToRot = set()
+        fresh = set()
+        
+        for i in range(len(grid)):
+            for j in range((len(grid[0]))):
+                if grid[i][j]==2:
+                    rotten.add((i, j))
+                elif grid[i][j]==1:
+                    fresh.add((i, j))
+        
+        while rotten:
+            i0, j0 = rotten.pop() #randomly get one
+            grid[i0][j0] = 2
+            
+            for i, j in ((i0+1, j0), (i0-1, j0), (i0, j0+1), (i0, j0-1)):
+                if i<0 or j<0 or i>=len(grid) or j>=len(grid[0]): continue
+                if (i, j) in rotten or (i, j) in aboutToRot: continue
+                if (i, j) in fresh:
+                    fresh.remove((i, j))
+                    aboutToRot.add((i, j))
+            
+            if not rotten and not aboutToRot: break
+            
+            if not rotten:
+                time += 1
+                rotten = aboutToRot
+                aboutToRot = set()
+                
+        if fresh: return -1
+        
+        return time
\ No newline at end of file
diff --git a/problems/python3/same-tree.py b/problems/python3/same-tree.py
new file mode 100755
index 0000000..6687f04
--- /dev/null
+++ b/problems/python3/same-tree.py
@@ -0,0 +1,16 @@
+"""
+Time: O(N)
+Space: O(LogN) if the tree is balanced.
+"""
+class Solution:
+    def isSameTree(self, node1: Optional[TreeNode], node2: Optional[TreeNode]) -> bool:
+        if not node1 and not node2:
+            return True
+
+        if (not node1 and node2) or (node1 and not node2):
+            return False
+
+        if node1.val!=node2.val:
+            return False
+
+        return self.isSameTree(node1.left, node2.left) and self.isSameTree(node1.right, node2.right)
\ No newline at end of file
diff --git a/problems/python3/search-a-2d-matrix.py b/problems/python3/search-a-2d-matrix.py
new file mode 100644
index 0000000..2afe301
--- /dev/null
+++ b/problems/python3/search-a-2d-matrix.py
@@ -0,0 +1,22 @@
+class Solution:
+    def searchMatrix(self, matrix: List[List[int]], target: int) -> bool:
+        N = len(matrix)
+        M = len(matrix[0])
+        
+        l = 0
+        r = N*M-1
+        
+        while l<=r:
+            m = l + int((r-l)/2)
+            
+            i = int(m/M)
+            j = m%M
+            
+            if matrix[i][j]<target:
+                l = m+1
+            elif matrix[i][j]>target:
+                r = m-1
+            else:
+                return True
+        
+        return False
\ No newline at end of file
diff --git a/problems/python3/serialize-and-deserialize-binary-tree.py b/problems/python3/serialize-and-deserialize-binary-tree.py
new file mode 100644
index 0000000..1b057d0
--- /dev/null
+++ b/problems/python3/serialize-and-deserialize-binary-tree.py
@@ -0,0 +1,22 @@
+class Codec:
+
+    def serialize(self, root):
+        if not root: return '#'
+        return str(root.val)+','+self.serialize(root.left)+','+self.serialize(root.right)
+        
+
+    def deserialize(self, data):
+        def helper():
+            if data[self.i]=='#':
+                self.i += 1
+                return None
+            
+            node = TreeNode(int(data[self.i]))
+            self.i += 1
+            node.left = helper()
+            node.right = helper()
+            return node
+        
+        data = data.split(",")
+        self.i = 0
+        return helper()
\ No newline at end of file
diff --git a/problems/python3/set-matrix-zeroes.py b/problems/python3/set-matrix-zeroes.py
new file mode 100644
index 0000000..dd836e5
--- /dev/null
+++ b/problems/python3/set-matrix-zeroes.py
@@ -0,0 +1,28 @@
+class Solution:
+    def setZeroes(self, matrix: List[List[int]]) -> None:
+        M = len(matrix)
+        N = len(matrix[0])
+        firstRowZero = False
+        
+        for i in range(M):
+            for j in range(N):
+                if matrix[i][j]==0:
+                    matrix[0][j] = 0                    
+                    if i==0:
+                        firstRowZero = True
+                    else:
+                        matrix[i][0] = 0
+        
+        for i in range(1, M):
+            if matrix[i][0]==0:
+                for j in range(N):
+                    matrix[i][j] = 0
+        
+        for j in range(N):
+            if matrix[0][j]==0:
+                for i in range(M):
+                    matrix[i][j] = 0
+        
+        if firstRowZero:
+            for j in range(N):
+                matrix[0][j] = 0
\ No newline at end of file
diff --git a/problems/python3/single-number.py b/problems/python3/single-number.py
new file mode 100644
index 0000000..daa3268
--- /dev/null
+++ b/problems/python3/single-number.py
@@ -0,0 +1,15 @@
+"""
+^ (XOR)
+The same will be 0
+0^0 = 0
+1^1 = 0
+
+Different will be 1
+1^0 = 1
+0^1 = 1
+"""
+class Solution:
+    def singleNumber(self, nums: List[int]) -> int:
+        ans = 0
+        for num in nums: ans ^= num
+        return ans
\ No newline at end of file
diff --git a/problems/python3/sliding-window-maximum.py b/problems/python3/sliding-window-maximum.py
new file mode 100644
index 0000000..a535c16
--- /dev/null
+++ b/problems/python3/sliding-window-maximum.py
@@ -0,0 +1,18 @@
+class Solution:
+    def maxSlidingWindow(self, nums: List[int], k: int) -> List[int]:
+        ans = []
+        q = collections.deque()
+        l = r = 0
+        
+        while r<len(nums):
+            while q and nums[q[-1]]<nums[r]: q.pop()
+            q.append(r)
+            
+            while q and q[0]<l: q.popleft()
+            
+            while r-l+1==k:
+                ans.append(nums[q[0]])
+                l += 1
+            
+            r += 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/spiral-matrix.py b/problems/python3/spiral-matrix.py
new file mode 100644
index 0000000..ddce35c
--- /dev/null
+++ b/problems/python3/spiral-matrix.py
@@ -0,0 +1,90 @@
+"""
+Original's solution
+Time: O(MN)
+Space: O(1)
+"""
+class Solution:
+    def spiralOrder(self, matrix: List[List[int]]) -> List[int]:
+        ans = []
+        x0 = 0
+        y0 = 0
+        dx = len(matrix[0])-1
+        dy = len(matrix)-1
+        direction = 'right'
+        isFirst = True
+        
+        ans.append(matrix[x0][y0])
+        while True:
+            if direction=='right':
+                for x in range(x0+1, x0+dx+1):
+                    ans.append(matrix[y0][x])
+                x0 += dx
+                direction = 'down'
+                
+                if isFirst:
+                    isFirst = False
+                else:
+                    dx -= 1
+                
+                if dy==0: break
+            
+            elif direction=='left':
+                for x in range(x0-1, x0-dx-1, -1):
+                    ans.append(matrix[y0][x])
+                x0 -= dx
+                direction = 'up'
+                dx -= 1
+                if dy==0: break
+            
+            elif direction=='down':
+                for y in range(y0+1, y0+dy+1):
+                    ans.append(matrix[y][x0])
+                y0 += dy
+                direction = 'left'
+                dy -= 1
+                if dx==0: break
+            
+            elif direction=='up':
+                for y in range(y0-1, y0-dy-1, -1):
+                    ans.append(matrix[y][x0])
+                y0 -= dy
+                direction = 'right'
+                dy -= 1
+                if dx==0: break
+        return ans
+
+
+"""
+Answer from Neetcode, more elegant.
+left, right, top, bottom is the border (index is exclusive on the border.
+In other words, for matrix[i][j]
+i: top<i<bottom
+j: left<j<right
+"""
+class Solution:
+    def spiralOrder(self, matrix: List[List[int]]) -> List[int]:
+        res = []
+        left, right = 0, len(matrix[0])
+        top, bottom = 0, len(matrix)
+
+        while left < right and top < bottom:
+            # get every i in the top row
+            for i in range(left, right):
+                res.append(matrix[top][i])
+            top += 1
+            # get every i in the right col
+            for i in range(top, bottom):
+                res.append(matrix[i][right - 1])
+            right -= 1
+            if not (left < right and top < bottom):
+                break
+            # get every i in the bottom row
+            for i in range(right - 1, left - 1, -1):
+                res.append(matrix[bottom - 1][i])
+            bottom -= 1
+            # get every i in the left col
+            for i in range(bottom - 1, top - 1, -1):
+                res.append(matrix[i][left])
+            left += 1
+
+        return res
\ No newline at end of file
diff --git a/problems/python3/subsets-ii.py b/problems/python3/subsets-ii.py
new file mode 100644
index 0000000..be0b0b7
--- /dev/null
+++ b/problems/python3/subsets-ii.py
@@ -0,0 +1,23 @@
+"""
+Time: O(N * 2^N)
+Space: O(N)
+"""
+class Solution:
+    def subsetsWithDup(self, nums: List[int]) -> List[List[int]]:
+        def helper(i):
+            if i==len(nums):
+                ans.append(subset.copy())
+                return
+            
+            subset.append(nums[i])
+            helper(i+1)
+            subset.pop()
+            
+            while i+1<len(nums) and nums[i]==nums[i+1]: i += 1
+            helper(i+1)
+            
+        nums.sort()
+        ans = []
+        subset = []
+        helper(0)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/subsets.py b/problems/python3/subsets.py
new file mode 100644
index 0000000..9cfcc00
--- /dev/null
+++ b/problems/python3/subsets.py
@@ -0,0 +1,22 @@
+"""
+Time: O(N * 2^N)
+Space: O(N) for "subset".
+"""
+class Solution:
+    def subsets(self, nums: List[int]) -> List[List[int]]:
+        def helper(i):
+            if not i<len(nums):
+                ans.append(subset.copy())
+                return
+            
+            subset.append(nums[i])
+            helper(i+1)
+            
+            subset.pop()
+            helper(i+1)
+        
+        ans = []
+        subset = []
+        
+        helper(0)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/substring-with-concatenation-of-all-words.py b/problems/python3/substring-with-concatenation-of-all-words.py
new file mode 100755
index 0000000..164dfa6
--- /dev/null
+++ b/problems/python3/substring-with-concatenation-of-all-words.py
@@ -0,0 +1,46 @@
+"""
+Time: O(W * N/W * W), there will be W time of iteration in the first loop ([0]), N/W of iteration in the second loop ([1]). In each loop, creating substring "word" and "popWord" takes O(W).
+Space: O(MW) for "wordSet".
+"""
+class Solution:
+    def findSubstring(self, s: str, words: List[str]) -> List[int]:
+        N = len(s)
+        M = len(words)
+        W = len(words[0])
+        wordSet = set(words)
+        ans = []
+        counter = collections.Counter(words)
+        
+        for i in range(W): #[0]
+            windowCounter = collections.Counter() #counter for the word in words
+            notInWords = 0 #number of word not in the wordSet
+            theSame = 0 #number of word with the same count with "counter"
+            j = i
+            
+            while j<N: #[1]
+                word = s[j:j+W]
+                if word in wordSet:
+                    windowCounter[word] += 1
+                    if windowCounter[word]==counter[word]:
+                        theSame += 1
+                    elif windowCounter[word]>counter[word]:
+                        theSame -= 1
+                else:
+                    notInWords += 1
+                    
+                popStart = j-M*W
+                if popStart>=0:
+                    popWord = s[popStart:popStart+W]
+                    if popWord in wordSet:
+                        windowCounter[popWord] -= 1
+                        if windowCounter[popWord]==counter[popWord]:
+                            theSame += 1
+                        elif windowCounter[popWord]<counter[popWord]:
+                            theSame -= 1
+                    else:
+                        notInWords -= 1
+                
+                if theSame==len(wordSet) and notInWords==0: ans.append(popStart+W)
+                j += W
+                
+        return ans
\ No newline at end of file
diff --git a/problems/python3/subtree-of-another-tree.py b/problems/python3/subtree-of-another-tree.py
new file mode 100755
index 0000000..7871f8c
--- /dev/null
+++ b/problems/python3/subtree-of-another-tree.py
@@ -0,0 +1,10 @@
+class Solution:
+    def isSubtree(self, root: Optional[TreeNode], subRoot: Optional[TreeNode]) -> bool:
+        if not root or not subRoot: return root==subRoot
+        if self.isSame(root, subRoot): return True
+        return self.isSubtree(root.left, subRoot) or self.isSubtree(root.right, subRoot)
+        
+    def isSame(self, p, q):
+        if not p or not q: return p==q
+        if p.val!=q.val: return False
+        return self.isSame(p.left, q.left) and self.isSame(p.right, q.right)
\ No newline at end of file
diff --git a/problems/python3/sum-of-two-integers.py b/problems/python3/sum-of-two-integers.py
new file mode 100644
index 0000000..25f16f9
--- /dev/null
+++ b/problems/python3/sum-of-two-integers.py
@@ -0,0 +1,12 @@
+"""
+Does not work with negative values yet.
+"""
+class Solution:
+    def getSum(self, a: int, b: int) -> int:
+        ans = a^b
+        carry = (a&b)<<1
+        
+        while carry!=0:
+            ans, carry = ans^carry, (ans&carry)<<1
+            
+        return ans
\ No newline at end of file
diff --git a/problems/python3/surrounded-regions.py b/problems/python3/surrounded-regions.py
new file mode 100644
index 0000000..42aa4c0
--- /dev/null
+++ b/problems/python3/surrounded-regions.py
@@ -0,0 +1,34 @@
+"""
+Time: O(N)
+Space: O(N)
+"""
+class Solution:
+    def solve(self, board: List[List[str]]) -> None:
+        def dfs(i0, j0):
+            if i0<0 or j0<0 or i0>=MAX_ROW or j0>=MAX_COL: return
+            if board[i0][j0]!='O': return
+            if (i0, j0) in survived: return
+            
+            survived.add((i0, j0))
+            dfs(i0+1, j0)
+            dfs(i0-1, j0)
+            dfs(i0, j0+1)
+            dfs(i0, j0-1)
+        
+        
+        MAX_ROW = len(board)
+        MAX_COL = len(board[0])
+        survived = set()
+        
+        for i in range(MAX_ROW):
+            dfs(i, 0)
+            dfs(i, MAX_COL-1)
+        
+        for j in range(MAX_COL):
+            dfs(0, j)
+            dfs(MAX_ROW-1, j)
+        
+        for i in range(MAX_ROW):
+            for j in range(MAX_COL):
+                board[i][j] = 'O' if (i, j) in survived else 'X'
+        return board
\ No newline at end of file
diff --git a/problems/python3/swim-in-rising-water.py b/problems/python3/swim-in-rising-water.py
new file mode 100644
index 0000000..dd5dbef
--- /dev/null
+++ b/problems/python3/swim-in-rising-water.py
@@ -0,0 +1,23 @@
+"""
+Time: O(N^2 * LogN^2) = O(N^2 * 2LogN) = O(N^2LogN), N is the number of elements in a row or column.
+Space: O(N^2)
+"""
+class Solution:
+    def swimInWater(self, grid: List[List[int]]) -> int:
+        ROWS = len(grid)
+        COLS = len(grid[0])
+        
+        visited = set()
+        h = [(grid[0][0], 0, 0)]
+        
+        while h:
+            t, r0, c0 = heapq.heappop(h)
+            
+            if (r0, c0) in visited: continue
+            visited.add((r0, c0))
+            if r0==ROWS-1 and c0==COLS-1: return t
+            
+            for r, c in ((r0+1, c0), (r0-1, c0), (r0, c0+1), (r0, c0-1)):
+                if r<0 or c<0 or r>=ROWS or c>=COLS: continue
+                if (r, c) in visited: continue
+                heapq.heappush(h, (max(t, grid[r][c]), r, c))
\ No newline at end of file
diff --git a/problems/python3/target-sum.py b/problems/python3/target-sum.py
new file mode 100644
index 0000000..832fadd
--- /dev/null
+++ b/problems/python3/target-sum.py
@@ -0,0 +1,25 @@
+"""
+Time: O(NS), S is sum(nums), N is len(nums). This is the max possible number of element in "history". Which will be lesser than 2^N.
+Space: O(NS)
+"""
+class Solution:
+    def findTargetSumWays(self, nums: List[int], target: int) -> int:
+        def dfs(i, curr):
+            #cache
+            if (i, curr) in history:
+                return history[(i, curr)]
+            
+            #ending condition
+            if i==len(nums):
+                if curr==target:
+                    history[(i, curr)] = 1
+                else:
+                    history[(i, curr)] = 0
+                return history[(i, curr)]
+            
+            history[(i, curr)] = dfs(i+1, curr+nums[i])+dfs(i+1, curr-nums[i])
+            return history[(i, curr)]
+        
+        ans = 0
+        history = {}
+        return dfs(0, 0)
\ No newline at end of file
diff --git a/problems/python3/task-scheduler.py b/problems/python3/task-scheduler.py
new file mode 100644
index 0000000..ddc4625
--- /dev/null
+++ b/problems/python3/task-scheduler.py
@@ -0,0 +1,23 @@
+class Solution:
+    def leastInterval(self, tasks: List[str], n: int) -> int:
+        q = collections.deque()
+        h = []
+        time = 0
+        
+        counter = collections.Counter(tasks)
+        for task in counter:
+            heapq.heappush(h, (-counter[task], task))
+        
+        while h or q:
+            if q and q[0][0]<=time:
+                _, count, task = q.popleft()
+                heapq.heappush(h, (-count, task))
+            
+            if h:
+                count, task = heapq.heappop(h)
+                count*=-1
+                count -= 1
+                if count>0: q.append((time+n+1, count, task))
+            time += 1
+        
+        return time
\ No newline at end of file
diff --git a/problems/python3/time-based-key-value-store.py b/problems/python3/time-based-key-value-store.py
new file mode 100644
index 0000000..f43facf
--- /dev/null
+++ b/problems/python3/time-based-key-value-store.py
@@ -0,0 +1,28 @@
+class TimeMap:
+
+    def __init__(self):
+        self.data = collections.defaultdict(list)
+
+    def set(self, key: str, value: str, timestamp: int) -> None:
+        self.data[key].append((timestamp, value))
+        
+    def get(self, key: str, timestamp: int) -> str:
+        dataList = self.data[key]
+        l = 0
+        r = len(dataList)-1
+        ans = ''
+        
+        while l<=r:
+            m = l + int((r-l)/2)
+            t = dataList[m][0]
+            
+            if t<timestamp:
+                ans = dataList[m][1]
+                l = m+1
+            elif t>timestamp:
+                r = m-1
+            else:
+                ans = dataList[m][1]
+                break
+        
+        return ans
\ No newline at end of file
diff --git a/problems/python3/top-k-frequent-elements.py b/problems/python3/top-k-frequent-elements.py
new file mode 100644
index 0000000..7d728cb
--- /dev/null
+++ b/problems/python3/top-k-frequent-elements.py
@@ -0,0 +1,53 @@
+#Bucket Sort
+class Solution:
+    def topKFrequent(self, nums: List[int], k: int) -> List[int]:
+        bucket = collections.defaultdict(list)
+        counter = collections.Counter(nums)
+        ans = []
+        
+        for num in counter:
+            bucket[counter[num]].append(num)
+        
+        tempCount = len(nums)
+        while len(ans)<k:
+            ans.extend(bucket[tempCount])
+            tempCount -= 1
+        
+        return ans
+
+#QuickSelect
+class Solution:
+    def topKFrequent(self, nums: List[int], K: int) -> List[int]:
+        def quickSelect(freqs, s, e, K):
+            i = s
+            t = s
+            j = e
+            pivot = freqs[(s+e)//2][1]
+            
+            while t<=j:
+                if freqs[t][1]<pivot:
+                    freqs[i], freqs[t] = freqs[t], freqs[i]
+                    i += 1
+                    t +=1
+                elif freqs[t][1]>pivot:
+                    freqs[j], freqs[t] = freqs[t], freqs[j]
+                    j -= 1
+                else:
+                    t += 1
+            if e-j>=K:
+                return quickSelect(freqs, j+1, e, K)
+            elif e-(i-1)>=K:
+                return pivot
+            else:
+                return quickSelect(freqs, s, i-1, K-(e-i+1))
+                
+        counts = collections.Counter(nums)
+        freqs = [(num, counts[num]) for num in counts]
+        ans = []
+        
+        KthLargestFreq = quickSelect(freqs, 0, len(freqs)-1, K)
+        
+        for num, freq in freqs:
+            if freq>=KthLargestFreq:
+                ans.append(num)
+        return ans
\ No newline at end of file
diff --git a/problems/python3/trapping-rain-water.py b/problems/python3/trapping-rain-water.py
new file mode 100644
index 0000000..fe01411
--- /dev/null
+++ b/problems/python3/trapping-rain-water.py
@@ -0,0 +1,35 @@
+"""
+The water stored at i is `min(leftMax, rightMax) - height[i]`
+leftMax: max height left of i
+rightMax: max height right of i
+
+To use only constant space.
+We can calculete the leftMax or rightMax and update ans along the way.
+And we always go with the side where leftMax or rightMax is smaller.
+
+Time: O(N)
+Space: O(1)
+"""
+class Solution:
+    def trap(self, height: List[int]) -> int:
+        ans = 0
+        l = 0
+        r = len(height)-1
+        
+        leftMax = float('-inf')
+        rightMax = float('-inf')
+        
+        while l<=r:
+            if leftMax<rightMax:
+                if height[l]>=leftMax:
+                    leftMax = height[l]
+                else:
+                    ans += leftMax-height[l]
+                l += 1
+            else:
+                if height[r]>=rightMax:
+                    rightMax = height[r]
+                else:
+                    ans += rightMax-height[r]
+                r -= 1
+        return ans
\ No newline at end of file
diff --git a/problems/python3/two-sum-ii-input-array-is-sorted.py b/problems/python3/two-sum-ii-input-array-is-sorted.py
new file mode 100644
index 0000000..2890404
--- /dev/null
+++ b/problems/python3/two-sum-ii-input-array-is-sorted.py
@@ -0,0 +1,11 @@
+class Solution:
+    def twoSum(self, numbers: List[int], target: int) -> List[int]:
+        i = 0
+        j = len(numbers)-1
+        
+        while numbers[i]+numbers[j] != target:
+            if numbers[i]+numbers[j] > target:
+                j -= 1
+            else:
+                i += 1
+        return (i+1, j+1)
\ No newline at end of file
diff --git a/problems/python3/two-sum.py b/problems/python3/two-sum.py
new file mode 100644
index 0000000..aa90d66
--- /dev/null
+++ b/problems/python3/two-sum.py
@@ -0,0 +1,8 @@
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        #needed: {number required : counter part index}
+        needed = {}
+        for i, num in enumerate(nums):
+            if num in needed:
+                return (needed[num], i)
+            needed[target-num] = i
\ No newline at end of file
diff --git a/problems/python3/unique-paths.py b/problems/python3/unique-paths.py
new file mode 100644
index 0000000..9c7ce40
--- /dev/null
+++ b/problems/python3/unique-paths.py
@@ -0,0 +1,11 @@
+class Solution:
+    def uniquePaths(self, m: int, n: int) -> int:
+        m = m-1 #number of steps need to move down
+        n = n-1 #number of steps need to move right
+        
+        #the total combination of m and n to sort will be (m+n)!
+        #since all "move down" are consider the same, we need to remove the repeatition of it sorting: m!.
+        #since all "move right" are consider the same, we need to remove the repeatition of it sorting: n!.
+        #(m+n)!/m!n!
+        
+        return math.factorial(m+n)//(math.factorial(m)*math.factorial(n))
\ No newline at end of file
diff --git a/problems/python3/valid-anagram.py b/problems/python3/valid-anagram.py
new file mode 100644
index 0000000..6c37cb0
--- /dev/null
+++ b/problems/python3/valid-anagram.py
@@ -0,0 +1,17 @@
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        counter = collections.Counter()
+        
+        for c in s:
+            counter[c] += 1
+        
+        for c in t:
+            if c not in counter:
+                return False
+            counter[c] -= 1
+        
+        for c in counter:
+            if counter[c]>0:
+                return False
+        
+        return True
\ No newline at end of file
diff --git a/problems/python3/valid-palindrome.py b/problems/python3/valid-palindrome.py
new file mode 100644
index 0000000..0069c83
--- /dev/null
+++ b/problems/python3/valid-palindrome.py
@@ -0,0 +1,15 @@
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        i = 0
+        j = len(s)-1
+        
+        while i<=j:
+            while i<j and not s[i].isalnum(): i += 1
+            while i<j and not s[j].isalnum(): j -= 1
+            
+            if s[i].lower()!=s[j].lower(): return False
+            
+            i += 1
+            j -= 1
+        
+        return True
\ No newline at end of file
diff --git a/problems/python3/valid-parentheses.py b/problems/python3/valid-parentheses.py
new file mode 100644
index 0000000..cb7d047
--- /dev/null
+++ b/problems/python3/valid-parentheses.py
@@ -0,0 +1,17 @@
+class Solution:
+    def isValid(self, s: str) -> bool:
+        mapping = {')': '(', ']': '[', '}':'{'}
+        stack = []
+        
+        for c in s:
+            if c not in mapping:
+                #open parentheses
+                stack.append(c)
+            else:
+                #close parentheses
+                if stack and stack[-1]==mapping[c]:
+                    stack.pop()
+                else:
+                    return False
+        
+        return not stack
\ No newline at end of file
diff --git a/problems/python3/valid-parenthesis-string.py b/problems/python3/valid-parenthesis-string.py
new file mode 100644
index 0000000..52ea446
--- /dev/null
+++ b/problems/python3/valid-parenthesis-string.py
@@ -0,0 +1,23 @@
+class Solution:
+    def checkValidString(self, s: str) -> bool:
+        leftMin = 0
+        leftMax = 0
+        
+        for c in s:
+            if c=='(':
+                leftMin += 1
+                leftMax += 1
+            elif c==')':
+                leftMin -= 1
+                leftMax -= 1
+            else:
+                leftMin -= 1
+                leftMax += 1
+            
+            if leftMax<0:
+                return False
+            
+            if leftMin<0:
+                leftMin = 0
+        
+        return leftMin == 0
\ No newline at end of file
diff --git a/problems/python3/valid-sudoku.py b/problems/python3/valid-sudoku.py
new file mode 100644
index 0000000..b2e6fa7
--- /dev/null
+++ b/problems/python3/valid-sudoku.py
@@ -0,0 +1,37 @@
+class Solution:
+    def isValidSudoku(self, board: List[List[str]]) -> bool:
+        def rowIsValid(i):
+            used = set()
+            for j in range(N):
+                if board[i][j]=='.': continue
+                if board[i][j] in used: return False
+                used.add(board[i][j])
+            return True
+        
+        def columnIsValid(i):
+            used = set()
+            for j in range(N):
+                if board[j][i]=='.': continue
+                if board[j][i] in used: return False
+                used.add(board[j][i])
+            return True
+        
+        def isBoxValid(i1, i2, j1, j2):
+            used = set()
+            for i in range(i1, i2+1):
+                for j in range(j1, j2+1):
+                    if board[i][j]=='.': continue
+                    if board[i][j] in used: return False
+                    used.add(board[i][j])
+            return True
+                
+        N = 9
+        for i in range(N):
+            if not rowIsValid(i): return False
+            if not columnIsValid(i): return False
+        
+        for i in range(0, N, 3):
+            for j in range(0, N, 3):
+                if not isBoxValid(i, i+2, j, j+2): return False
+        
+        return True
\ No newline at end of file
diff --git a/problems/python3/validate-binary-search-tree.py b/problems/python3/validate-binary-search-tree.py
new file mode 100755
index 0000000..4185bc6
--- /dev/null
+++ b/problems/python3/validate-binary-search-tree.py
@@ -0,0 +1,24 @@
+"""
+The inorder travsersal of a BST is always increasing.
+
+Time: O(N)
+Space: O(N)
+"""
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        lastVal = float('-inf')
+        stack = []
+        
+        node = root
+        while stack or node:
+            while node:
+                stack.append(node)
+                node = node.left
+            
+            node = stack.pop()
+            
+            if not lastVal<node.val: return False
+            lastVal = node.val
+            
+            node = node.right
+        return True
\ No newline at end of file
diff --git a/problems/python3/walls-and-gates.py b/problems/python3/walls-and-gates.py
new file mode 100644
index 0000000..5f27f9a
--- /dev/null
+++ b/problems/python3/walls-and-gates.py
@@ -0,0 +1,29 @@
+class Solution:
+    def wallsAndGates(self, rooms: List[List[int]]) -> None:
+        def bfs(i0, j0) -> None:
+            q = collections.deque([(i0+1, j0, 1), (i0-1, j0, 1), (i0, j0+1, 1), (i0, j0-1, 1)])
+            visited = set()
+            
+            while q:
+                i, j, dis = q.popleft()
+                
+                if i<0 or j<0 or i>=MAX_ROW or j>=MAX_COL: continue
+                if rooms[i][j]==0 or rooms[i][j]==-1: continue
+                if (i, j, dis) in visited: continue
+                visited.add((i, j, dis))
+                
+                if dis<rooms[i][j]:
+                    rooms[i][j] = dis
+                    q.append((i+1, j, dis+1))
+                    q.append((i-1, j, dis+1))
+                    q.append((i, j+1, dis+1))
+                    q.append((i, j-1, dis+1))
+                
+        MAX_ROW = len(rooms)
+        MAX_COL = len(rooms[0])
+        
+        for i in range(MAX_ROW):
+            for j in range(MAX_COL):
+                if rooms[i][j]==0:
+                    bfs(i, j)
+        return rooms
\ No newline at end of file
diff --git a/problems/python3/word-break.py b/problems/python3/word-break.py
new file mode 100644
index 0000000..8d2c817
--- /dev/null
+++ b/problems/python3/word-break.py
@@ -0,0 +1,23 @@
+"""
+Time: O(N^2 * M). N is the length of the s. M is the number of word in wordDict.
+Note that s[i:i+len(word)]==word takes O(N) time.
+
+Space: O(N) for the recursion memory stack size.
+
+dfs(i) := will return starting at index i, if i to the end the string can be separated.
+"""
+class Solution:
+    def wordBreak(self, s: str, wordDict: List[str]) -> bool:
+        def dfs(i):
+            if i==len(s): return True
+            if i in history and not history[i]: return False
+            
+            for word in wordDict:
+                if i+len(word)<=len(s) and s[i:i+len(word)]==word:
+                    history[i] = True
+                    if dfs(i+len(word)): return True
+            history[i] = False
+            return False
+        
+        history = {}
+        return dfs(0)
\ No newline at end of file
diff --git a/problems/python3/word-ladder.py b/problems/python3/word-ladder.py
new file mode 100644
index 0000000..453fb4a
--- /dev/null
+++ b/problems/python3/word-ladder.py
@@ -0,0 +1,40 @@
+"""
+Time: O(NxM^2). N is the number of words. M is the length of the word.
+Note that, getPatterns() takes O(M^2) since creating new string will also takes O(M) and for each word we do that O(M) times.
+
+Space: O(NxM^2)
+"""
+class Solution:
+    def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
+        def getPatterns(word) -> List[str]:
+            patterns = []
+            for i in range(len(word)):
+                pattern = word[:i]+'*'+word[i+1:]
+                patterns.append(pattern)
+            return patterns
+        
+        
+        if endWord not in wordList: return 0
+        wordList.append(beginWord)
+        
+        #build adjacency list
+        nei = collections.defaultdict(list)
+        for word in wordList:
+            for pattern in getPatterns(word):
+                nei[pattern].append(word)
+        
+        #BFS
+        q = collections.deque([(beginWord, 1)])
+        visited = set()
+        while q:
+            word, steps = q.popleft()
+            
+            if word in visited: continue
+            visited.add(word)
+            if word==endWord: return steps
+            
+            for pattern in getPatterns(word):
+                for nextWord in nei[pattern]:
+                    if nextWord in visited: continue
+                    q.append((nextWord, steps+1))
+        return 0
\ No newline at end of file
diff --git a/problems/python3/word-search-ii.py b/problems/python3/word-search-ii.py
new file mode 100644
index 0000000..a285655
--- /dev/null
+++ b/problems/python3/word-search-ii.py
@@ -0,0 +1,54 @@
+"""
+Time: O(M * 4 * 3^(L-1)), M is board elements count, L is the average word length.
+Space: O(N), N is the number of char in the words
+
+This solution will TLE. We need to further "prune" the trie once we reach the leaf node.
+"""
+class TrieNode():
+    def __init__(self):
+        self.children = {}
+        self.isEnd = False
+    def addWord(self, word):
+        curr = self
+        for c in word:
+            if c not in curr.children:
+                curr.children[c] = TrieNode()
+            curr = curr.children[c]
+        curr.isEnd = True
+    
+class Solution:
+    def findWords(self, board: List[List[str]], words: List[str]) -> List[str]:
+        def dfs(r, c, node, word):
+            if c<0 or r<0 or c==COLS or r==ROWS: return
+            if board[r][c] not in node.children: return
+            if (r, c) in visited: return
+            
+            visited.add((r, c))
+            node = node.children[board[r][c]]
+            word += board[r][c]
+            if node.isEnd:
+                ans.add(word)
+                node.isEnd = False
+                
+            dfs(r+1, c, node, word)
+            dfs(r-1, c, node, word)
+            dfs(r, c+1, node, word)
+            dfs(r, c-1, node, word)
+            
+            visited.remove((r, c))
+        
+        
+        root = TrieNode()
+        for word in words:
+            root.addWord(word)
+            
+        ROWS = len(board)
+        COLS = len(board[0])
+        
+        visited = set()
+        ans = set()
+        node = root
+        for r in range(ROWS):
+            for c in range(COLS):
+                dfs(r, c, root, '')
+        return ans
\ No newline at end of file
diff --git a/problems/python3/word-search.py b/problems/python3/word-search.py
new file mode 100644
index 0000000..b7ebb09
--- /dev/null
+++ b/problems/python3/word-search.py
@@ -0,0 +1,27 @@
+class Solution:
+    def exist(self, board: List[List[str]], word: str) -> bool:
+        def helper(x, i, j):
+            if i<0 or i>=N or j<0 or j>=M: return False
+            if (i, j) in usedWords: return False
+            usedWords.add((i, j))
+            
+            if word[x]==board[i][j]:
+                if x==len(word)-1:
+                    return True
+                else:
+                    if helper(x+1, i+1, j): return True
+                    if helper(x+1, i, j+1): return True
+                    if helper(x+1, i-1, j): return True
+                    if helper(x+1, i, j-1): return True
+                    
+                    
+            usedWords.remove((i, j))
+            return False
+                    
+        usedWords = set()
+        N = len(board)
+        M = len(board[0])
+        for i in range(N):
+            for j in range(M):
+                if helper(0, i, j): return True
+        return False
\ No newline at end of file
diff --git a/problems/vertical-order-traversal-of-a-binary-tree.py b/problems/vertical-order-traversal-of-a-binary-tree.py
deleted file mode 100644
index 9cc1849..0000000
--- a/problems/vertical-order-traversal-of-a-binary-tree.py
+++ /dev/null
@@ -1,28 +0,0 @@
-"""
-The description wants us to report from left to right, top to bottom.
-Thus, we keep add value into `temp` with `x_position` as the key and `(y_position, node.val)` as the value.
-And maintain `min_x`, `max_x` at the same time. So we know how to iterate the `temp`.
-"""
-from collections import defaultdict
-
-class Solution(object):
-    def verticalTraversal(self, root):
-        temp = defaultdict(list)
-        min_x = 0
-        max_x = 0
-
-        stack = []
-        stack.append((root, 0, 0))
-        while stack:
-            node, x, y = stack.pop()
-            temp[x].append((y, node.val)) #append the value of height, so we can sort by height later on
-            min_x = min(min_x, x)
-            max_x = max(max_x, x)
-            if node.left: stack.append((node.left, x-1, y+1))
-            if node.right: stack.append((node.right, x+1, y+1))
-        
-        opt = []
-        for i in range(min_x, max_x+1):
-            opt.append([v for y, v in sorted(temp[i])]) #the temp[i] will be sorted by y_position then sorted by node.val
-        
-        return opt
\ No newline at end of file