该题是要找出树的最大深度，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 private int max = 0;
12 public int maxDepth(TreeNode root) {
13
14 helper(root, 0);
15
16 return max;
17 }
18
19 private void helper(TreeNode root, int tmp){
20
21 if(root == null){
22 if(tmp > max){
23 max = tmp;
24 }
25 return ;
26 }
27
28 tmp ++;
29 helper(root.left, tmp);
30 helper(root.right, tmp);
31 }
32 }
END