diff --git a/src/main/ts/g0001_0100/s0092_reverse_linked_list_ii/solution.ts b/src/main/ts/g0001_0100/s0092_reverse_linked_list_ii/solution.ts
index 41c7ae8..f19791c 100644
--- a/src/main/ts/g0001_0100/s0092_reverse_linked_list_ii/solution.ts
+++ b/src/main/ts/g0001_0100/s0092_reverse_linked_list_ii/solution.ts
@@ -35,10 +35,10 @@ function reverseBetween(head: ListNode | null, left: number, right: number): Lis
         prev1 = tail
         tail = next
     }
-    if (prev !== null) {
-        prev.next = prev1
-    } else {
+    if (prev === null) {
         head = prev1
+    } else {
+        prev.next = prev1
     }
     if (start !== null) {
         start.next = tail
diff --git a/src/main/ts/g0001_0100/s0097_interleaving_string/solution.ts b/src/main/ts/g0001_0100/s0097_interleaving_string/solution.ts
index 255f12f..c33db43 100644
--- a/src/main/ts/g0001_0100/s0097_interleaving_string/solution.ts
+++ b/src/main/ts/g0001_0100/s0097_interleaving_string/solution.ts
@@ -5,7 +5,7 @@ function isInterleave(s1: string, s2: string, s3: string): boolean {
     if (s3.length !== s1.length + s2.length) {
         return false
     }
-    const cache: boolean[][] = Array.from({ length: s1.length + 1 }, () => Array(s2.length + 1).fill(null))
+    const cache: boolean[][] = Array.from({ length: s1.length + 1 }, () => new Array(s2.length + 1).fill(null))
     return isInterleaveHelper(s1, s2, s3, 0, 0, 0, cache)
 }
 
diff --git a/src/main/ts/g0101_0200/s0101_symmetric_tree/solution.ts b/src/main/ts/g0101_0200/s0101_symmetric_tree/solution.ts
index ebb3065..0606517 100644
--- a/src/main/ts/g0101_0200/s0101_symmetric_tree/solution.ts
+++ b/src/main/ts/g0101_0200/s0101_symmetric_tree/solution.ts
@@ -33,8 +33,7 @@ function isSymmetric(root: TreeNode | null): boolean {
             if (!leftNode || !rightNode || leftNode.val != rightNode.val) {
                 return false
             }
-            queue.push([leftNode.left, rightNode.right])
-            queue.push([leftNode.right, rightNode.left])
+            queue.push([leftNode.left, rightNode.right], [leftNode.right, rightNode.left])
         }
     }
     return true
diff --git a/src/main/ts/g0101_0200/s0103_binary_tree_zigzag_level_order_traversal/solution.ts b/src/main/ts/g0101_0200/s0103_binary_tree_zigzag_level_order_traversal/solution.ts
index 5f57a79..a795149 100644
--- a/src/main/ts/g0101_0200/s0103_binary_tree_zigzag_level_order_traversal/solution.ts
+++ b/src/main/ts/g0101_0200/s0103_binary_tree_zigzag_level_order_traversal/solution.ts
@@ -18,37 +18,31 @@ import { TreeNode } from '../../com_github_leetcode/treenode'
  * }
  */
 function zigzagLevelOrder(root: TreeNode | null): number[][] {
+    if (!root) return []
+
     const result: number[][] = []
-    if (root === null) {
-        return result
-    }
-    const q: (TreeNode | null)[] = [root, null]
-    let zig = true
-    let level: number[] = []
-    while (q.length > 0) {
-        const node = q.shift()
-        if (node === null) {
-            result.push(level)
-            zig = !zig
-            level = []
-            if (q.length > 0) {
-                q.push(null)
-            }
-        } else {
-            if (zig) {
-                level.push(node.val)
-            } else {
-                level.unshift(node.val)
-            }
-            if (node.left !== null) {
-                q.push(node.left)
-            }
-            if (node.right !== null) {
-                q.push(node.right)
-            }
+    const queue: TreeNode[] = [root]
+    let leftToRight = true
+
+    while (queue.length) {
+        const size = queue.length
+        const level = new Array<number>(size)
+
+        for (let i = 0; i < size; i++) {
+            const node = queue.shift()!
+            const index = leftToRight ? i : size - 1 - i
+            level[index] = node.val
+
+            if (node.left) queue.push(node.left)
+            if (node.right) queue.push(node.right)
         }
+
+        result.push(level)
+        leftToRight = !leftToRight
     }
+
     return result
 }
 
+
 export { zigzagLevelOrder }
diff --git a/src/main/ts/g0101_0200/s0120_triangle/solution.ts b/src/main/ts/g0101_0200/s0120_triangle/solution.ts
index d3e8a6d..ae1563b 100644
--- a/src/main/ts/g0101_0200/s0120_triangle/solution.ts
+++ b/src/main/ts/g0101_0200/s0120_triangle/solution.ts
@@ -8,7 +8,7 @@ function minimumTotal(triangle: number[][]): number {
     }
     const rows = triangle.length
     const cols = triangle[rows - 1].length
-    const dp: number[][] = Array.from({ length: rows }, () => Array(cols).fill(-10001))
+    const dp: number[][] = Array.from({ length: rows }, () => new Array(cols).fill(-10001))
     function dfs(row: number, col: number): number {
         if (row >= triangle.length) {
             return 0
diff --git a/src/main/ts/g0101_0200/s0125_valid_palindrome/solution.ts b/src/main/ts/g0101_0200/s0125_valid_palindrome/solution.ts
index 81df05d..920c5e6 100644
--- a/src/main/ts/g0101_0200/s0125_valid_palindrome/solution.ts
+++ b/src/main/ts/g0101_0200/s0125_valid_palindrome/solution.ts
@@ -5,7 +5,7 @@ function isPalindrome(s: string): boolean {
     if (s.length < 2) {
         return true
     }
-    let sFormated = s.toLowerCase().replace(/[^a-zA-Z0-9]/g, '')
+    let sFormated = s.toLowerCase().replaceAll(/[^a-zA-Z0-9]/g, '')
     let reversed = sFormated.split('').reverse().join('').replace(',', '')
     return sFormated === reversed
 }
diff --git a/src/main/ts/g0101_0200/s0127_word_ladder/solution.ts b/src/main/ts/g0101_0200/s0127_word_ladder/solution.ts
index 521c4eb..bcff9fa 100644
--- a/src/main/ts/g0101_0200/s0127_word_ladder/solution.ts
+++ b/src/main/ts/g0101_0200/s0127_word_ladder/solution.ts
@@ -3,41 +3,40 @@
 // #2025_04_07_Time_41_ms_(95.63%)_Space_63.50_MB_(82.78%)
 
 function ladderLength(beginWord: string, endWord: string, wordList: string[]): number {
-    const wordSet: Set<string> = new Set(wordList)
-    if (!wordSet.has(endWord)) {
-        return 0
-    }
-    let beginSet: Set<string> = new Set([beginWord])
-    let endSet: Set<string> = new Set([endWord])
-    const visited: Set<string> = new Set()
-    let len = 1
-    const wordLen = beginWord.length
-    while (beginSet.size > 0 && endSet.size > 0) {
-        if (beginSet.size > endSet.size) {
-            ;[beginSet, endSet] = [endSet, beginSet]
-        }
-        const tempSet: Set<string> = new Set()
-        for (const word of beginSet) {
-            const chars = word.split('')
-            for (let i = 0; i < wordLen; i++) {
-                const oldChar = chars[i]
-                for (let c = 97; c <= 122; c++) {
-                    chars[i] = String.fromCharCode(c)
-                    const nextWord = chars.join('')
-                    if (endSet.has(nextWord)) {
-                        return len + 1
-                    }
-                    if (!visited.has(nextWord) && wordSet.has(nextWord)) {
-                        tempSet.add(nextWord)
-                        visited.add(nextWord)
+    const wordSet = new Set(wordList)
+    if (!wordSet.has(endWord)) return 0
+
+    const queue = [beginWord]
+    let steps = 1
+
+    while (queue.length > 0) {
+        const levelSize = queue.length
+
+        for (let i = 0; i < levelSize; i++) {
+            const word = queue.shift()!
+
+            // Try changing each letter
+            for (let pos = 0; pos < word.length; pos++) {
+                for (let charCode = 97; charCode <= 122; charCode++) {
+                    // 'a' to 'z'
+                    const newChar = String.fromCodePoint(charCode)
+                    if (newChar === word[pos]) continue
+
+                    const newWord = word.slice(0, pos) + newChar + word.slice(pos + 1)
+
+                    if (newWord === endWord) return steps + 1
+
+                    if (wordSet.has(newWord)) {
+                        queue.push(newWord)
+                        wordSet.delete(newWord)
                     }
                 }
-                chars[i] = oldChar
             }
         }
-        beginSet = tempSet
-        len++
+
+        steps++
     }
+
     return 0
 }
 
diff --git a/src/main/ts/g0101_0200/s0130_surrounded_regions/solution.ts b/src/main/ts/g0101_0200/s0130_surrounded_regions/solution.ts
index db3fb81..57b52bf 100644
--- a/src/main/ts/g0101_0200/s0130_surrounded_regions/solution.ts
+++ b/src/main/ts/g0101_0200/s0130_surrounded_regions/solution.ts
@@ -6,47 +6,42 @@
  Do not return anything, modify board in-place instead.
  */
 function solve(board: string[][]): void {
-    if (board.length === 0) {
-        return
-    }
+    if (!board.length) return
+
     const rows = board.length
     const cols = board[0].length
-    const dfs = (board: string[][], row: number, col: number): void => {
-        if (row < 0 || row >= rows || col < 0 || col >= cols || board[row][col] !== 'O') {
-            return
-        }
-        board[row][col] = '#'
-        dfs(board, row + 1, col)
-        dfs(board, row - 1, col)
-        dfs(board, row, col + 1)
-        dfs(board, row, col - 1)
-    }
-    for (let i = 0; i < cols; i++) {
-        if (board[0][i] === 'O') {
-            dfs(board, 0, i)
-        }
-        if (board[rows - 1][i] === 'O') {
-            dfs(board, rows - 1, i)
+    const dirs = [[1,0], [-1,0], [0,1], [0,-1]]
+
+    const dfs = (r: number, c: number): void => {
+        if (
+            r < 0 || r >= rows ||
+            c < 0 || c >= cols ||
+            board[r][c] !== 'O'
+        ) return
+
+        board[r][c] = '#'
+        for (const [dr, dc] of dirs) {
+            dfs(r + dr, c + dc)
         }
     }
+
+    // mark border-connected O's
     for (let i = 0; i < rows; i++) {
-        if (board[i][0] === 'O') {
-            dfs(board, i, 0)
-        }
-        if (board[i][cols - 1] === 'O') {
-            dfs(board, i, cols - 1)
-        }
+        dfs(i, 0)
+        dfs(i, cols - 1)
     }
+    for (let j = 0; j < cols; j++) {
+        dfs(0, j)
+        dfs(rows - 1, j)
+    }
+
+    // flip & restore
     for (let i = 0; i < rows; i++) {
         for (let j = 0; j < cols; j++) {
-            if (board[i][j] === 'O') {
-                board[i][j] = 'X'
-            }
-            if (board[i][j] === '#') {
-                board[i][j] = 'O'
-            }
+            board[i][j] = board[i][j] === '#' ? 'O' : 'X'
         }
     }
 }
 
+
 export { solve }
diff --git a/src/main/ts/g0101_0200/s0138_copy_list_with_random_pointer/solution.ts b/src/main/ts/g0101_0200/s0138_copy_list_with_random_pointer/solution.ts
index 0e9542c..c6506bd 100644
--- a/src/main/ts/g0101_0200/s0138_copy_list_with_random_pointer/solution.ts
+++ b/src/main/ts/g0101_0200/s0138_copy_list_with_random_pointer/solution.ts
@@ -38,7 +38,7 @@ const copyRandomList = (head: Node | null): Node | null => {
     while (cursor !== null && newCursor !== null) {
         if (cursor.random !== null) {
             const targetNode = map.get(cursor.random)
-            if (typeof targetNode !== 'undefined') {
+            if (targetNode !== undefined) {
                 newCursor.random = targetNode
             }
         }
diff --git a/src/main/ts/g0101_0200/s0173_binary_search_tree_iterator/solution.ts b/src/main/ts/g0101_0200/s0173_binary_search_tree_iterator/solution.ts
index 077a102..4e3dd67 100644
--- a/src/main/ts/g0101_0200/s0173_binary_search_tree_iterator/solution.ts
+++ b/src/main/ts/g0101_0200/s0173_binary_search_tree_iterator/solution.ts
@@ -27,7 +27,11 @@ class BSTIterator {
     next(): number {
         let res = -1
         while (this.node !== null) {
-            if (this.node.left !== null) {
+            if (this.node.left === null) {
+                res = this.node.val
+                this.node = this.node.right
+                return res
+            } else {
                 let rightMost = this.node.left
                 while (rightMost.right !== null && rightMost.right !== this.node) {
                     rightMost = rightMost.right
@@ -42,10 +46,6 @@ class BSTIterator {
                     this.node = this.node.right
                     return res
                 }
-            } else {
-                res = this.node.val
-                this.node = this.node.right
-                return res
             }
         }
         return res
diff --git a/src/main/ts/g0201_0300/s0208_implement_trie_prefix_tree/solution.ts b/src/main/ts/g0201_0300/s0208_implement_trie_prefix_tree/solution.ts
index f490b91..29d84c7 100644
--- a/src/main/ts/g0201_0300/s0208_implement_trie_prefix_tree/solution.ts
+++ b/src/main/ts/g0201_0300/s0208_implement_trie_prefix_tree/solution.ts
@@ -69,7 +69,7 @@ class Trie {
     }
 
     private getCharIndex(char: string): number {
-        return char.charCodeAt(0) - 'a'.charCodeAt(0)
+        return char.codePointAt(0)! - 'a'.codePointAt(0)!
     }
 }
 
diff --git a/src/main/ts/g0201_0300/s0211_design_add_and_search_words_data_structure/solution.ts b/src/main/ts/g0201_0300/s0211_design_add_and_search_words_data_structure/solution.ts
index 60d11ae..3274412 100644
--- a/src/main/ts/g0201_0300/s0211_design_add_and_search_words_data_structure/solution.ts
+++ b/src/main/ts/g0201_0300/s0211_design_add_and_search_words_data_structure/solution.ts
@@ -14,8 +14,7 @@ class WordDictionary {
 
     addWord(word: string): void {
         let current = this.root
-        for (let i = 0; i < word.length; i++) {
-            const letter = word[i]
+        for (const letter of word) {
             if (!current.hasOwnProperty(letter)) {
                 current[letter] = {}
             }
@@ -33,12 +32,10 @@ class WordDictionary {
             index += 1
             if (currentLetter === '.') {
                 return Object.keys(subtree).some((letter) => searchSubtree(word, index, subtree[letter] as TrieNode))
+            } else if (subtree.hasOwnProperty(currentLetter)) {
+                return searchSubtree(word, index, subtree[currentLetter] as TrieNode)
             } else {
-                if (subtree.hasOwnProperty(currentLetter)) {
-                    return searchSubtree(word, index, subtree[currentLetter] as TrieNode)
-                } else {
-                    return false
-                }
+                return false
             }
         }
         return searchSubtree(word, 0, this.root)
diff --git a/src/main/ts/g0201_0300/s0224_basic_calculator/solution.ts b/src/main/ts/g0201_0300/s0224_basic_calculator/solution.ts
index 811c88f..ece2f51 100644
--- a/src/main/ts/g0201_0300/s0224_basic_calculator/solution.ts
+++ b/src/main/ts/g0201_0300/s0224_basic_calculator/solution.ts
@@ -5,34 +5,37 @@ function calculate(s: string): number {
     let i = 0
 
     function helper(ca: string[]): number {
+        let result = 0
         let num = 0
-        let prenum = 0
-        let isPlus = true
+        let sign = 1
+
         while (i < ca.length) {
             const c = ca[i]
-            if (c !== ' ') {
-                if (c >= '0' && c <= '9') {
-                    num = num * 10 + parseInt(c)
-                } else if (c === '+') {
-                    prenum += num * (isPlus ? 1 : -1)
-                    isPlus = true
-                    num = 0
-                } else if (c === '-') {
-                    prenum += num * (isPlus ? 1 : -1)
-                    isPlus = false
-                    num = 0
-                } else if (c === '(') {
-                    i++
-                    num = helper(ca)
-                } else if (c === ')') {
-                    prenum += num * (isPlus ? 1 : -1)
-                    return prenum
-                }
+
+            if (c === ' ') {
+                i++
+                continue
+            }
+
+            if (c >= '0' && c <= '9') {
+                num = num * 10 + (c.codePointAt(0)! - 48)
+            } else if (c === '+' || c === '-') {
+                result += sign * num
+                sign = c === '+' ? 1 : -1
+                num = 0
+            } else if (c === '(') {
+                i++
+                num = helper(ca)
+            } else if (c === ')') {
+                return result + sign * num
             }
+
             i++
         }
-        return prenum + num * (isPlus ? 1 : -1)
+
+        return result + sign * num
     }
+
     return helper(s.split(''))
 }
 
diff --git a/src/main/ts/g0201_0300/s0228_summary_ranges/solution.ts b/src/main/ts/g0201_0300/s0228_summary_ranges/solution.ts
index 347c551..3ff94f1 100644
--- a/src/main/ts/g0201_0300/s0228_summary_ranges/solution.ts
+++ b/src/main/ts/g0201_0300/s0228_summary_ranges/solution.ts
@@ -10,7 +10,9 @@ function summaryRanges(nums: number[]): string[] {
     let b = a
     let strB = ''
     for (let i = 1; i < n; i++) {
-        if (nums[i] !== b + 1) {
+        if (nums[i] === b + 1) {
+            b++
+        } else {
             strB = a.toString()
             if (a !== b) {
                 strB += '->' + b.toString()
@@ -18,9 +20,6 @@ function summaryRanges(nums: number[]): string[] {
             ranges.push(strB)
             a = nums[i]
             b = a
-            strB = ''
-        } else {
-            b++
         }
     }
     strB = a.toString()
diff --git a/src/main/ts/g0201_0300/s0230_kth_smallest_element_in_a_bst/solution.ts b/src/main/ts/g0201_0300/s0230_kth_smallest_element_in_a_bst/solution.ts
index bc18228..db2fcae 100644
--- a/src/main/ts/g0201_0300/s0230_kth_smallest_element_in_a_bst/solution.ts
+++ b/src/main/ts/g0201_0300/s0230_kth_smallest_element_in_a_bst/solution.ts
@@ -26,10 +26,10 @@ function kthSmallest(root: TreeNode | null, k: number): number {
             while (rightmost.right !== null) rightmost = rightmost.right
             rightmost.right = cur
             cur = left
-        } else if (--k !== 0) {
-            cur = cur.right
-        } else {
+        } else if (--k === 0) {
             break
+        } else {
+            cur = cur.right
         }
     }
     return cur.val
diff --git a/src/main/ts/g0201_0300/s0242_valid_anagram/solution.ts b/src/main/ts/g0201_0300/s0242_valid_anagram/solution.ts
index 936a93e..716b940 100644
--- a/src/main/ts/g0201_0300/s0242_valid_anagram/solution.ts
+++ b/src/main/ts/g0201_0300/s0242_valid_anagram/solution.ts
@@ -8,8 +8,8 @@ function isAnagram(s: string, t: string): boolean {
     }
     let counts = new Array(26).fill(0)
     for (let i = 0; i < s.length; ++i) {
-        counts[s.charCodeAt(i) - 'a'.charCodeAt(0)]++
-        counts[t.charCodeAt(i) - 'a'.charCodeAt(0)]--
+        counts[s.codePointAt(i)! - 'a'.codePointAt(0)!]++
+        counts[t.codePointAt(i)! - 'a'.codePointAt(0)!]--
     }
     return counts.every((c) => c === 0)
 }
diff --git a/src/main/ts/g0201_0300/s0290_word_pattern/solution.ts b/src/main/ts/g0201_0300/s0290_word_pattern/solution.ts
index 4bc7476..fd2bb15 100644
--- a/src/main/ts/g0201_0300/s0290_word_pattern/solution.ts
+++ b/src/main/ts/g0201_0300/s0290_word_pattern/solution.ts
@@ -15,10 +15,8 @@ function wordPattern(pattern: string, s: string): boolean {
                 return false
             }
             map.set(char, word)
-        } else {
-            if (map.get(char) !== word) {
-                return false
-            }
+        } else if (map.get(char) !== word) {
+            return false
         }
     }
     return true
diff --git a/src/main/ts/g0301_0400/s0347_top_k_frequent_elements/solution.ts b/src/main/ts/g0301_0400/s0347_top_k_frequent_elements/solution.ts
index d925d9e..9501a6c 100644
--- a/src/main/ts/g0301_0400/s0347_top_k_frequent_elements/solution.ts
+++ b/src/main/ts/g0301_0400/s0347_top_k_frequent_elements/solution.ts
@@ -7,8 +7,11 @@ function topKFrequent(nums: number[], k: number): number[] {
     let hashMap = new Map<number, number>()
     let res: number[] = new Array()
     nums.forEach((num) => {
-        if (!hashMap.has(num)) hashMap.set(num, 1)
-        else hashMap.set(num, hashMap.get(num)! + 1)
+        if (hashMap.has(num)) {
+            hashMap.set(num, hashMap.get(num)! + 1)
+        } else {
+            hashMap.set(num, 1)
+        }
     })
     hashMap.forEach((value, key) => {
         if (elementCount[value] === undefined) elementCount[value] = [key]