LeetCode 437. Path Sum III (STL map前缀和)

找遍所有路径，特判以根为起点的串即可。

代码：

/** * 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 {public:    int ans = 0;    map <int, int> m;    int pathSum(TreeNode* root, int sum) {         findSum(root, sum, 0);        return ans;    }    void findSum(TreeNode *node, int sum, int num){        if(node == NULL) return;        num += node->val;        if(num == sum) ans += m[0]+1;        else ans += m[num-sum];        m[num]++;        findSum(node->left, sum, num);        findSum(node->right, sum, num);        m[num]--;    }};