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

/**
* 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]--;
}

};