[LC] 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],

    3
   / \
  9  20
    /  \
   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         return helper(root);
13     }
14     
15     private int helper(TreeNode root) {
16         if (root == null) {
17             return 0;
18         }
19         int left = helper(root.left);
20         int right = helper(root.right);
21         return Math.max(left, right) + 1;
22     }
23 }