[LeetCode]508. Most Frequent Subtree Sum

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本文介绍了解决LeetCode上第508题Most Frequent Subtree Sum的方法。通过递归遍历二叉树并使用哈希表记录每个子树的和及其出现频率，最终找出出现最频繁的子树和。

[LeetCode]508. Most Frequent Subtree Sum

题目描述

这里写图片描述

思路

递归，保存每次的和到map中，同时计算和最多出现的次数
思路简单，回顾树的操作

代码

#include <iostream>
#include <vector>
#include <unordered_map>
using namespace std;


 struct TreeNode {
     int val;
     TreeNode *left;
     TreeNode *right;
     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 };

 int countSubtreeSums(TreeNode *r, unordered_map<int, int> &countSum, int& max){
     if (!r)
         return 0;
     int sum = r->val;
     sum += countSubtreeSums(r->left, countSum, max);
     sum += countSubtreeSums(r->right, countSum, max);
     ++countSum[sum];
     if (countSum[sum] > max)
         max = countSum[sum];
     return sum;
 }

 vector<int> findFrequentTreeSum(TreeNode* root) {
     unordered_map<int, int> countSum;
     int max = 0;
     countSubtreeSums(root, countSum, max);

     vector<int> res;
     for (const auto &x : countSum){
         if (max == x.second)
             res.push_back(x.first);
     }
     return res;
 }

int main(){
    int depth = 4;
    TreeNode *root = new TreeNode(5);
    TreeNode *left = new TreeNode(2);
    TreeNode *right = new TreeNode(-3);
    root->left = left;
    root->right = right;

    vector<int> res = findFrequentTreeSum(root);
    for (auto &p : res){
        cout << p << endl;
    }
    system("pause");
}