CheckBSTIterative

import java.util.*;                           //In-order traversal
class Node                                    //Time complexity is O(n)
{
	int data;
	Node left, right;
	public Node(int item)
	{
		data = item;
		left = right = null;
	}
}

public class BinaryTree
{
	Node root;
	Node prev;

	boolean isBST() {
		prev = null;
		return isValidBST(root);
	}

    boolean isValidBST(Node node) {
        if(root == null) return false;
 
        Stack<Node> S = new Stack<>();
        int first_compare = 1; 
        
        while(node!=null){
        S.push(node);
        node = node.left;
        }
        
        while(!S.isEmpty()){
                node = S.pop();
                if(prev !=null && node.data <= prev.data){
                    return false;
                }
                prev = node;
                if (node.right != null) {
                node = node.right;
                 
                while (node != null) {
                    S.push(node);
                    node = node.left;
                }
            }
        }
     return true;
     }

	public static void main(String args[])
	{
		BinaryTree tree = new BinaryTree();
		tree.root = new Node(4);
		tree.root.left = new Node(2);
		tree.root.right = new Node(5);
		tree.root.left.left = new Node(10);
		tree.root.left.right = new Node(3);
		if (tree.isBST())
			System.out.println("IS BST");
		else
			System.out.println("Not a BST");
	}
}