508. Most Frequent Subtree Sum

「爱情、让人受尽委屈。」 2024-04-08 09:28 52阅读 0赞

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).

**Example 1:**

![5e7e2898d6ffa1d9a55132dae4ff9a95.jpeg][]

Input: root = [5,2,-3]
    Output: [2,-3,4]

**Example 2:**

![f3d4218ae5679b4c9eecb9d675f91c3f.jpeg][]

Input: root = [5,2,-5]
    Output: [2]

题目：给定一个二叉树，找出所有出现次数最多的子树和的值

思路：DFS，用一个hashmap来保存所有字数和出现的次数，代码：

/**
     * Definition for a binary tree node.
     * struct TreeNode {
     *     int val;
     *     TreeNode *left;
     *     TreeNode *right;
     *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
     *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
     *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
     * };
     */
    class Solution {
    public:
        int getSum(TreeNode* node, unordered_map<int, int>& m, int& maxFre){
            if(!node) return 0;
            int left = getSum(node->left, m, maxFre);
            int right = getSum(node->right, m, maxFre);
            int sum = node->val + left + right;
            m[sum]++;
            if(m[sum] > maxFre) maxFre = m[sum];
            return sum;
        }
        vector<int> findFrequentTreeSum(TreeNode* root) {
            unordered_map<int, int> m;
            int maxFre = 0;
            getSum(root, m, maxFre);
            vector<int> res;
            for(auto mp : m){
                if(mp.second == maxFre)
                    res.push_back(mp.first);
            }
            return res;
        }
    };

[5e7e2898d6ffa1d9a55132dae4ff9a95.jpeg]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/08/7f6406c63f5d458d822565cdc6df8752.jpeg
[f3d4218ae5679b4c9eecb9d675f91c3f.jpeg]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/08/31f403c87cfb45668ca76c8d446faf8e.jpeg