using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

    public static Node CommonAncestor(Node root, Node node1, Node node2)

    {

        if (node1 == node2)

            return node1;

        if (node1 == root || node2 == root)

            return root;

        var isNode1Left = Covers(root.Left, node1);

        var isNode2Left = Covers(root.Left, node2);

        if (isNode1Left != isNode2Left)

            return root;

        if (isNode1Left)

            return CommonAncestor(root.Left, node1, node2);

        else

            return CommonAncestor(root.Right, node1, node2);

    }

    private static bool Covers(Node root, Node node)

    {

        if (node == null)

            return false;

        if (root == node)

        {

            return true;

        }

        return Covers(root.Left, node) || Covers(root.Right, node); 

    }

}

​

public class Node

{

    public Node Parent { get; set; }

    public Node Left { get; set; }

    public Node Right { get; set; }

}