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.

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool hasPathSum(TreeNode *root, int sum) {
13         if(!root) return false;
14         if(!root->left && !root->right) return sum == root->val;
15         return hasPathSum(root->left, sum-root->val) || 
16             hasPathSum(root->right, sum-root->val);
17     }
18 };