Hard ·

LeetCode 32: Longest Valid Parentheses

2026-11-03·1 分鐘

LeetCode Hard String Dynamic-Programming Stack Bracket-Sequences

LeetCode 32 解題筆記：堆疊存索引，底部放目前這段合法括號開始前的位置，長度是 i - stack[-1]

LeetCode 25: Reverse Nodes in k-Group

2026-11-01·1 分鐘

LeetCode Hard Linked-List Recursion

LeetCode 25 解題筆記：先數長度決定要翻幾組，每組翻 k 個節點；dummy head 讓第一組不用特判，翻完原本的頭變成這組的尾

LeetCode 834: Sum of Distances in Tree

2026-10-31·2 分鐘

LeetCode Hard Dynamic-Programming Tree Depth-First-Search Graph Dp-on-Trees

LeetCode 834 解題筆記：換根 DP：先以 0 為根算子樹大小和 ans[0]，再用 ans[child] = ans[parent] - size + (n - size) 推到每個節點

LeetCode 460: LFU Cache

2026-10-31·2 分鐘

LeetCode Hard Hash-Table Linked-List Design Doubly-Linked-List

LeetCode 460 解題筆記：key → 節點、freq → 雙向鏈結串列，再記一個 min_freq；同頻率裡用 LRU 決定淘汰誰

LeetCode 2858: Minimum Edge Reversals So Every Node Is Reachable

2026-10-31·1 分鐘

LeetCode Hard Dynamic-Programming Depth-First-Search Breadth-First-Search Graph

LeetCode 2858 解題筆記：換根 DP：先算以 0 為根要翻幾條邊，根移到小孩時，這條邊原本順向就 +1、逆向就 -1

LeetCode 968: Binary Tree Cameras

2026-10-30·1 分鐘

LeetCode Hard Dynamic-Programming Tree Depth-First-Search Binary-Tree Dp-on-Trees

LeetCode 968 解題筆記：後序貪心，每個節點三種狀態：有相機、被覆蓋、沒被覆蓋；有小孩沒被覆蓋才放相機，最後根沒被覆蓋要補一台

LeetCode 297: Serialize and Deserialize Binary Tree

2026-10-25·1 分鐘

LeetCode Hard String Tree Depth-First-Search Breadth-First-Search Design Binary-Tree

LeetCode 297 解題筆記：前序遍歷加上空節點標記就能唯一還原樹；反序列化用迭代器依序取值

LeetCode 124: Binary Tree Maximum Path Sum

2026-10-24·1 分鐘

LeetCode Hard Dynamic-Programming Tree Depth-First-Search Binary-Tree Dp-on-Trees

LeetCode 124 解題筆記：DFS 回傳往下的一條鏈，在每個節點用「左鏈 + 自己 + 右鏈」更新答案；負的鏈要丟掉

LeetCode 23: Merge k Sorted Lists

2026-10-20·3 分鐘

LeetCode Hard Linked-List Heap Merge-Sort Divide-and-Conquer

LeetCode 23 解題筆記：heap 只放 k 個開頭，以及分治兩兩合併

LeetCode 493: Reverse Pairs

2026-10-19·3 分鐘

LeetCode Hard Merge-Sort Divide-and-Conquer Two-Pointers Counting

LeetCode 493 解題筆記：計數條件和排序條件不同時，先掃再合併

LeetCode 4051: Count Subarrays with Distant Sums

2026-10-19·4 分鐘

LeetCode Hard Merge-Sort Prefix-Sum Binary-Indexed-Tree Biweekly-Contest

LeetCode 4051 解題筆記：前綴和 + 補集 + 區間計數，以及 k = 0 的邊界

LeetCode 2426: Number of Pairs Satisfying Inequality

2026-10-19·2 分鐘

LeetCode Hard Merge-Sort Divide-and-Conquer Two-Pointers Counting

LeetCode 2426 解題筆記：先化簡式子，再判斷符合的是前綴還是後綴