[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.

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 class Solution {
     public int maxDepth(TreeNode root) {
         return helper(root);
     }

     private int helper(TreeNode root) {
         if (root == null) {
             return 0;
         }
         int left = helper(root.left);
         int right = helper(root.right);
         return Math.max(left, right) + 1;
     }
 }