https://leetcode.com/problems/path-sum-iii/description/

class Solution {public:    int pathSum(TreeNode* root, int sum) {        unordered_map
     
       sums;        sums[0] = 1;        return dfs(root, sums, sum, 0);    }    int dfs(TreeNode* root, unordered_map
      
       & sums, int target, int curSum) {        int res = 0;        if (root == NULL)   return res;        curSum += root->val;        res += sums[curSum-target];                sums[curSum]++;        res += dfs(root->left, sums, target, curSum);        res += dfs(root->right, sums, target, curSum);        sums[curSum]--;                return res;    }};