### 题目

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.

### 回溯递归

#### 代码

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean hasPathSum(TreeNode root, int sum) {
if (root == null) { return false; }
int remain = sum - root.val;
if (root.left == null && root.right == null && remain == 0) { return true; }
if (root.left != null && hasPathSum(root.left,remain)) { return true; }
if (root.right != null && hasPathSum(root.right,remain)) { return true; }
return false;
}
}


#### 稍微优化代码

public class Solution {
public boolean hasPathSum(TreeNode root, int sum) {
if (root == null) { return false; }
if (root.left == null && root.right == null) { return sum == root.val; }
int remain = sum - root.val;
return hasPathSum(root.left,remain) || hasPathSum(root.right,remain);
}
}