思路：前缀和+递归+回溯

  1.     unordered_map<int, int> m;
  2.     int ans = 0;
  3.     int pathSum(TreeNode* root, int sum) {
  4.         if(!root)return 0;
  5.         m[0] = 1;
  6.         dfs(root, sum, 0);
  7.         return ans;
  8.     }
  9.     void dfs(TreeNode *node, int sum, int num){
  10.         if(!node)return;
  11.         num += node->val;
  12.         if(m.count(num - sum))ans += m[num - sum];
  13.         ++m[num];
  14.         dfs(node->left, sum, num);
  15.         dfs(node->right, sum, num);
  16.         --m[num];
  17.     }