using System;

​

public class Program

{

    public static void Main()

    {

        var tree = new BinaryTree<int>();

        tree.Left = new BinaryTree<int>();

        tree.Left.Left = new BinaryTree<int>();

        tree.Right = new BinaryTree<int>();

        tree.Right.Right = new BinaryTree<int>();

        tree.Right.Right.Left = new BinaryTree<int>();

        Console.WriteLine(tree.NodeCount());

    }

}

​

public interface IBinaryTree<T>

{

    public IBinaryTree<T> Left { get; set;}

​

    public IBinaryTree<T> Right { get; set;}

​

    public T Value { get; }

}

​

public static class BinaryTreeExtentions

{

    public static int NodeCount<T>(this IBinaryTree<T> tree)

    {

        throw new NotImplementedException();

    }

}

​

public class BinaryTree<T> : IBinaryTree<T>

{

    public IBinaryTree<T> Left { get; set; }

​

    public IBinaryTree<T> Right { get; set; }

​

    public T Value { get; }

}