4 Sum

applewjg · applewjg · commit bbf328355c8b · 2014-12-22T17:56:08.000+08:00
Change-Id: I3ecede96a036e6b2682aefdff4b7aacc1bd3bc5a
diff --git a/3SumClosest.java b/3SumClosest.java
@@ -0,0 +1,38 @@
+/*
+ Author:     King, wangjingui@outlook.com
+ Date:       Dec 20, 2014
+ Problem:    3Sum Closest
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/3sum-closest/
+ Notes:
+ Given an array S of n integers, find three integers in S such that the sum is closest to 
+ a given number, target. Return the sum of the three integers. 
+ You may assume that each input would have exactly one solution.
+ For example, given array S = {-1 2 1 -4}, and target = 1.
+ The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).
+
+ Solution: Similar to 3Sum, taking O(n^2) time complexity.
+ */
+
+public class Solution {
+    public int threeSumClosest(int[] num, int target) {
+        int N = num.length;
+        if (N < 3) return 0;
+        int res = num[0] + num[1] + num[2];
+        Arrays.sort(num);
+        for (int i = 0; i < N-2; ++i)
+        {
+            int l = i + 1, r = N - 1;
+            while (l < r)
+            {
+                int threesum = num[i] + num[l] + num[r];
+                if (threesum == target) return target;
+                else if (threesum < target) ++l;
+                else --r;
+                if (Math.abs(threesum - target) < Math.abs(res - target))
+                    res = threesum;
+            }
+        }
+        return res;
+    }
+}
diff --git a/4Sum.java b/4Sum.java
@@ -0,0 +1,57 @@
+/*
+ Author:     King, wangjingui@outlook.com
+ Date:       Dec 20, 2014
+ Problem:    4Sum
+ Difficulty: Medium
+ Source:     https://oj.leetcode.com/problems/4sum/
+ Notes:
+ Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? 
+ Find all unique triplets in the array which gives the sum of zero.
+ Note:
+ Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? 
+ Find all unique quadruplets in the array which gives the sum of target.
+ Note:
+ Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a <= b <= c <= d)
+ The solution set must not contain duplicate quadruplets.
+ For example, given array S = {1 0 -1 0 -2 2}, and target = 0.
+ A solution set is:
+ (-1,  0, 0, 1)
+ (-2, -1, 1, 2)
+ (-2,  0, 0, 2)
+
+ Solution: Similar to 3Sum, 2Sum.
+ */
+
+ public class Solution {
+    public List<List<Integer>> fourSum(int[] num, int target) {
+        int N = num.length;
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        if (N < 4) return res;
+        Arrays.sort(num);
+        for (int i = 0; i < N; ++i)
+        {
+            if (i > 0 && num[i] == num[i-1]) continue; // avoid duplicates
+            for (int j = i+1; j < N; ++j)
+            {
+                if (j > i+1 && num[j] == num[j-1]) continue; // avoid duplicates
+                int twosum = target - num[i] - num[j];
+                int l = j + 1, r = N - 1;
+                while (l < r)
+                {
+                    int sum = num[l] + num[r];
+                    if (sum == twosum) {
+                    ArrayList<Integer> tmp = new ArrayList<Integer>();
+                    tmp.add(num[i]); tmp.add(num[j]); tmp.add(num[l]); tmp.add(num[r]);
+                    res.add(tmp);
+                        while (l < r && num[l+1] == num[l]) l++; // avoid duplicates
+                        while (l < r && num[r-1] == num[r]) r--; // avoid duplicates
+                        l++; r--;
+                    }
+                    else if (sum < twosum) l++;
+                    else r--;
+                }
+            }
+        }
+        return res;
+    }
+}
