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]--;
    }
    
};