using System;

public class Program

{

    public static void Main()

    {

        Node root = newNode(4);

        root.left = newNode(2);

        root.right = newNode(5);

        root.left.left = newNode(1);

        root.left.right = newNode(3);

​

        if (isBST(root,null,null))

            Console.Write("Is BST");

        else

            Console.Write("Not a BST");

    }

    public class Node

    {

        public int data;

        public Node left, right;

    };

​

    // Returns true if given tree is BST.

    static Boolean isBST(Node root, Node l, Node r)

    {

        // Base condition

        if (root == null)

            return true;

​

        // if left node exist then check it has

        // correct data or not i.e. left node's data

        // should be less than root's data

        if (l != null && root.data <= l.data)

            return false;

​

        // if right node exist then check it has

        // correct data or not i.e. right node's data

        // should be greater than root's data

        if (r != null && root.data >= r.data)

            return false;

​

        // check recursively for every node.

        return isBST(root.left, l, root) &&

            isBST(root.right, root, r);

    }

​

    // Helper function that allocates a new node with the

    //given data && null left && right pointers. /

    static Node newNode(int data)

    {

        Node node = new Node();

        node.data = data;

        node.left = node.right = null;

        return (node);

    }

}