Check if Binary Tree is BST

BSTCheck.java

class Node
{
	int data;
	Node left, right;

	public Node(int item)
	{
		data = item;
		left = right = null;
	}
}

public class BinaryTree
{
	// Root of the Binary Tree
	Node root;

	// To keep tract of previous node in Inorder Traversal
	Node prev;

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

	/* Returns true if given search tree is binary
	search tree (efficient version) */
	boolean isBST(Node node)
	{
		// traverse the tree in inorder fashion and
		// keep a track of previous node
		if (node != null)
		{
			if (!isBST(node.left))
				return false;

			// allows only distinct values node
			
			if (prev != null) {
			    System.out.println(" Prev: " + prev.data + " at node " + node.data);    
			}
			
			if (prev != null && node.data <= prev.data )
			{
				// System.out.println(" Prev: " + prev);
				return false;
			}	
			prev = node;
			return isBST(node.right);
		}
		return true;
	}

	/* Driver program to test above functions */
	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(1);
		tree.root.left.right = new Node(3);

		if (tree.isBST())
			System.out.println("IS BST");
		else
			System.out.println("Not a BST");
	}
}