437. Path Sum III

class Solution {
public:
    int pathSum(TreeNode* root, int sum) {
        if (!root) return 0;
		// DFS在确定的起点开始搜索计数，pathSum 用来更换地点
        return DFS(root, 0, sum)+pathSum(root->left, sum)+pathSum(root->right, sum);
    }
private:
    int DFS(TreeNode* root, int sum, int &target) {
        if (!root) return 0;
        int curSum=sum+root->val;
		// 相等即为1，然后再搜索左子树以及右子树进行递归计数
        return (curSum==target) + DFS(root->left, curSum, target) + DFS(root->right, curSum, target);
    }
};