LeetCode知识点总结 - 508

LeetCode 508. Most Frequent Subtree Sum

题目

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).

思路

用hashmap储存sum出现的次数，DFS所有subtree

答案

class Solution:
    def findFrequentTreeSum(self, root):
        if root is None: return []

        def dfs(node):
            if node is None: return 0
            s = node.val + dfs(node.left) + dfs(node.right)
            count[s] += 1
            return s

        count = collections.Counter()
        dfs(root)
        maxCount = max(count.values())
        return [s for s in count if count[s] == maxCount]
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