using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var tree = new TreeNode(4){

            left = new TreeNode(2) {

                left = new TreeNode(1),

                right = new TreeNode(3)

            },

            right = new TreeNode(6) {

                right = new TreeNode(8)

            }

        };

        var solution = new Solution();

        Console.WriteLine(solution.GetSizeOfTree(tree));

        Console.ReadLine();

    }

}

​

 public class TreeNode {

      public int val;

      public TreeNode left;

      public TreeNode right;

      public TreeNode(int x) { val = x; }

  }

​

public class Solution {

    public int GetSizeOfTree(TreeNode root) {

            if (root == null)

                return 0;

        var leftLevel = GetSizeOfTree(root.left);

        var rightLevel = GetSizeOfTree(root.right);

        return leftLevel + rightLevel + 1;

    }

}