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.

分析：用DFS解，代码如下：

class Solution {public:    bool hasPathSum(TreeNode *root, int sum) {        return dfs(root, 0, sum);    }        bool dfs(TreeNode *root, int cur_sum, int sum){        if(root == NULL) return false;        cur_sum += root->val;                if(cur_sum == sum && root->left == NULL && root->right == NULL) return true;                return dfs(root->left, cur_sum, sum) || dfs(root->right, cur_sum, sum);    }};

Leetcode:Path Sum