leetcode 112 —— Path Sum

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.
For example:
Given the below binary tree and 

sum
= 22

,

5
/ \
4   8
/   / \
11  13  4
/  \      \
7    2      1

return true, as there exist a root-to-leaf path 

5->4->11->2

 which sum is 22.

思路： 每到一个叶子节点，就进行判断。 设定flag用以判定和提前退出

class Solution {
private:
bool flag = false;
public:
bool hasPathSum(TreeNode* root, int sum) {
dfs(root, 0, sum);
return flag;
}
void dfs(TreeNode* root, int cnt, int& sum){
if (flag || !root)
return;
if (!root->left&&!root->right){ //叶子节点
if (cnt + root->val == sum) //到该节点的和等于sum
flag = true;
return;
}

dfs(root->left, cnt + root->val, sum);
dfs(root->right, cnt + root->val, sum);
}
};