508. Most Frequent Subtree Sum

Given the root of a tree, you are asked to find the most frequent subtree sum. 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). So what is the most frequent subtree sum value? If there is a tie, return all the values with the highest frequency in any order.

这道题其实就是采用深度优先搜索的算法，然后再用了Map来存储数据，这里要注意的是root对应的节点的值，这里用val表示。然后就是注意最后dfs，是if和if和if，因为任何存在的可能的和都得算一次。然后再回到原函数，把最大次数的和求出来，然后再把所有出现次数等于maxx的数压入vector，这里注意是把对应的和压入，而不是把次数压入。
代码如下：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {private:    map<int,int>m;public:    vector<int> findFrequentTreeSum(TreeNode* root) {        dfs(root);        vector<int>v;        int maxx=-100;        for(auto it=m.begin();it!=m.end();it++){            maxx=max(maxx,it->second);        }        for(auto it=m.begin();it!=m.end();it++){            if(maxx==it->second){                v.push_back(it->first);            }        }        return v;    }    void dfs(TreeNode* root){        if(root==NULL)return;        if(root->left!=NULL){            dfs(root->left);            root->val+=root->left->val;        }       if(root->right!=NULL){            dfs(root->right);            root->val+=root->right->val;        }        m[root->val]++;    }};