using System;

/* a class containning the left and right nodes and key value*/

public class Node

{

    public int data;

    public Node left,right;

    public Node(int item)

    {

    data = item;

    left=right=null;

    }

}

public class Program

{

    //the root of the node

    public Node root;

    //return true if the search tree is a binary search tree 

    public bool isBst 

    {

        get

        {

            return IsBstUtil(root,int.MinValue,int.MaxValue);

        }

    }

    // if the search tree is a binary search tree then the data will be >= min or <= max

​

    public bool IsBstUtil(Node node,int min,int max)

    {

        //return true if node is null

        if(node == null)

        {

            return true;

        }

        //it's violates the condition so it will return false

        if(node.data < min || node.data > max)

        {

            return false;

        }

        //otherwise check the subtrees recursively

        return (IsBstUtil(node.left,min,node.data-1) && IsBstUtil(node.right,min,node.data+1));

    }

    public static void Main()

    { 

        Program tree= new Program();

        tree.root = new Node(4);

        tree.root.left = new Node(2);

        tree.root.left.right = new Node(3);

        tree.root.left.left = new Node(1);

        tree.root.right = new Node(5);

          if(tree.isBst)

          {

              Console.WriteLine("Its a Binary tree");

          }

        else

        {

          Console.WriteLine("Its not a Binary tree");

        }

    }

}