Maximum Depth of Binary Tree

 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 public class Solution {
11     public int maxDepth(TreeNode root) {
12         if (root == null) {
13             return 0;
14         }
15         int result = 1;
16         return helper(root, result);
17     }
18     private int helper(TreeNode root, int result) {
19         if (root == null) {
20             return 0;
21         }
22         if (root.left == null && root.right == null) {
23             return result;
24         } else if (root.left != null && root.right == null) {
25             return helper(root.left, result + 1);
26         } else if (root.right != null && root.left == null) {
27             return helper(root.right, result + 1);
28         } else {
29             return Math.max(helper(root.left, result + 1), helper(root.right, result + 1));
30         }
31     }
32 }