TikTok 面试真题解析：最频繁子树和（Most Frequent Subtree Sum）｜二叉树 DFS 统计题

TikTok Interview Question: Most Frequent Subtree Sum in a Binary Tree

Given the root of a binary tree, return the most frequent subtree sum.
If there is a tie, return all the values with the highest frequency in any order.
The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself).

这道题的关键在于对每个节点计算“以它为根的子树节点值总和”，然后统计这些和出现的次数。因为子树和依赖左右子树的结果，通常使用后序遍历来计算。在递归过程中，用哈希表记录每个子树和的出现频率，遍历完成后再找出出现次数最多的那个（或多个）。本质是 DFS + 频率统计的结合。