这道题是要判断由根到树叶结点的值的和是否存在等于某个值。主要思路是对树进行前序遍历，依次将结点值相加，当到达树叶结点后直接判断是否等于指定值。
代码如下：
1 /**
2 * Definition for a binary tree node.
3 * public class TreeNode {
4 * int val;
5 * TreeNode left;
6 * TreeNode right;
7 * TreeNode(int x) { val = x; }
8 * }
9 */
10 class Solution {
11 public boolean hasPathSum(TreeNode root, int sum) {
12 return helper(root, 0, sum);
13 }
14
15 private boolean helper(TreeNode root, int tmpsum, int sum) {
16 if(root == null){
17 return false;
18 }
19
20 tmpsum += root.val;
21 if( root.left == null && root.right == null && tmpsum == sum ){
22 return true;
23 }
24
25 return helper(root.left, tmpsum, sum) || helper(root.right, tmpsum, sum);
26 }
27 }
END