Tree ·

LeetCode 2385: Amount of Time for Binary Tree to Be Infected

2026-11-01·1 分鐘

LeetCode Medium Hash-Table Tree Depth-First-Search Breadth-First-Search Binary-Tree

LeetCode 2385 解題筆記：把樹轉成無向圖，從 start 做 BFS，層數減一就是分鐘數

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 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 889: Construct Binary Tree from Preorder and Postorder Traversal

2026-10-29·1 分鐘

LeetCode Medium Array Hash-Table Divide-and-Conquer Tree Binary-Tree

LeetCode 889 解題筆記：stack 存目前這條路徑，堆頂等於 postorder 的下一個就代表子樹完成要 pop；新節點先接左邊

LeetCode 863: All Nodes Distance K in Binary Tree

2026-10-28·1 分鐘

LeetCode Medium Hash-Table Tree Depth-First-Search Breadth-First-Search Binary-Tree

LeetCode 863 解題筆記：先把樹轉成無向圖（補上往父節點的邊），再從 target BFS 走 k 步

LeetCode 437: Path Sum III

2026-10-28·1 分鐘

LeetCode Medium Tree Depth-First-Search Binary-Tree

LeetCode 437 解題筆記：樹上的前綴和 + 雜湊表：先查 prefix - target 再把自己加進去，離開節點時要扣回來

LeetCode 652: Find Duplicate Subtrees

2026-10-26·1 分鐘

LeetCode Medium Hash-Table Tree Depth-First-Search Binary-Tree

LeetCode 652 解題筆記：後序把每棵子樹編碼，出現第二次時加入答案；字串編碼最差 O(n^2)，改成 (值, 左 id, 右 id) 編號可到 O(n)

LeetCode 99: Recover Binary Search Tree

2026-10-25·1 分鐘

LeetCode Medium Tree Depth-First-Search Binary-Search-Tree Binary-Tree

LeetCode 99 解題筆記：中序遍歷找逆序：第一次逆序的前一個、最後一次逆序的後一個，就是被交換的兩個節點

LeetCode 450: Delete Node in a BST

2026-10-25·1 分鐘

LeetCode Medium Tree Binary-Search-Tree Binary-Tree

LeetCode 450 解題筆記：找到節點後分三種情況：沒有小孩、一個小孩直接接上，兩個小孩用右子樹最小值替換再刪掉它

LeetCode 449: Serialize and Deserialize BST

2026-10-25·1 分鐘

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

LeetCode 449 解題筆記：BST 只要存前序，不用空節點標記；反序列化帶著上下界遞迴，O(n)

LeetCode 331: Verify Preorder Serialization of a Binary Tree

2026-10-25·1 分鐘

LeetCode Medium String Stack Tree Binary-Tree

LeetCode 331 解題筆記：數空位：一開始 1 個，每個節點用掉 1 個，非空節點再多出 2 個；中途不能變負，最後要剛好 0

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