104. Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

return its depth = 3.

 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 int maxDepth(TreeNode root) {
12         if (root == null) return 0;
13         int d = 0;
14         if (root.left != null) {
15             d = Math.max(d, maxDepth(root.left));
16         }
17         if (root.right != null) {
18             d = Math.max(d, maxDepth(root.right));
19         }
20         return d + 1;
21     }
22 }