IsTreeBinarySearchTree.java

import java.util.*;

public class IsTreeBinarySearchTree {
 
    public static void main(String[] args) {
       // build a tree with Nodes...root is root Node

        System.out.println((isTreeBST(root) ? "Tree is BST!" : "Tree is NOT BST!"));
    }

    public static boolean isTreeBST(Node root) {
        return isTreeBSTUtil(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    public static boolean isTreeBSTUtil(Node root, int minVal, int maxVal) {

        // base case, below tree
        if (root == null) return true;

        if (root.data > minVal && root.data <= maxVal && 
                isTreeBSTUtil(root.left, minVal, root.data) &&
                    isTreeBSTUtil(root.right, root.data, maxVal))
            return true;
        else
            return false;
    }
}

class Node {

    int data;
    public Node left;
    public Node right;

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