using System;

public class Program

{

    public static void Main()

    {

        TreeNode root = new TreeNode(3);

        root.left = new TreeNode(9);

        root.right = new TreeNode(20);

        root.right.left = new TreeNode(15);

        root.right.right = new TreeNode(7);

        var result = MaxDepth(root);

        Console.WriteLine(result);

    }

    public static int MaxDepth(TreeNode root)

    {

        if (root == null) {return 0;}

        return 1 + Math.Max(MaxDepth(root.left), MaxDepth(root.right));

    }

}

​

public class TreeNode

{

    public int val; 

    public TreeNode left;

    public TreeNode right;

    public TreeNode(int val = 0, TreeNode left = null, TreeNode right = null)

    {

        this.val = val;

        this.left = left;

        this.right = right;

    }

}