543. Diameter of Binary Tree 用两条边的返回值计算更新全局变量，递归返回单边结果
124. Binary Tree Maximum Path Sum 跟543相似
687. Longest Univalue Path 同类题
337. House Robber III 这一系列都要分exclude和include两种情况考虑
112. Path Sum 简化的DFS，只判断是否存在解
113. Path Sum II 找到所有解，注意合并重复的代码！
437. Path Sum III 不需要root to leaf，map
110. Balanced Binary Tree 一旦找到解就不再继续搜索，返回值为-1代表false
111. Minimum Depth of Binary Tree
**572. Subtree of Another Tree ** 分解成左右子问题
**965. Univalued Binary Tree **分解成左右子问题
129. Sum Root to Leaf Numbers 分解成左右子问题 medium
**257. Binary Tree Paths **DFS
**236. Lowest Common Ancestor of a Binary Tree **
102. Binary Tree Level Order Traversal top down
**107. Binary Tree Level Order Traversal II **bottom up
**429. N-ary Tree Level Order Traversal ** 还是层次遍历
**1302. Deepest Leaves Sum **
872. Leaf-Similar Trees 把dfs写成generator的形式
树剪枝系列
814. Binary Tree Pruning 分解成左右子问题
669. Trim a Binary Search Tree 分解成左右子问题
**1325. Delete Leaves With a Given Value **
前中后序遍历
**94. Binary Tree Inorder Traversal **
144. Binary Tree Preorder Traversal
**589. N-ary Tree Preorder Traversal **
590. N-ary Tree Postorder Traversal
**979. Distribute Coins in Binary Tree ** flow代表移动次数
508. Most Frequent Subtree Sum
