Maximum Depth of Binary Tree
Given the root of a binary tree, return its maximum depth . A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Example 1: Input: root = [3,9,20,null,null,15,7] Output: 3 Example 2: Input: root = [1,null,2] Output: 2 Example 3: Input: root = [] Output: 0 Example 4: Input: root = [0] Output: 1 Constraints: The number of nodes in the tree is in the range [0, 10 4 ] . -100 <= Node.val <= 100 Solution 1: /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solu