Is it a binary tree

Questions please see HackerRank. It was too long.

In simple, how can you check if a tree is a binary search tree?

Idea

Read the binary search tree from leftmost to rightmost, and make sure the order is always increasing. If you still remember how to do an in-order recursive visit for a tree. Then it is not hard. The hard thing is that how you can code it elegantly.

Solution

/* Hidden stub code will pass a root argument to the function below. Complete the function to solve the challenge. Hint: you may want to write one or more helper functions.  

The Node class is defined as follows:
    class Node {
        int data;
        Node left;
        Node right;
     }
*/
    class IncrChecker {
        private Set visited = new HashSet();
        private int max = Integer.MIN_VALUE;
        public boolean isValid(int input) {
            if (visited.contains(input)) return false;
            if (input < max) return false;
            max = input;
            visited.add(input);
            return true;
        }
    }

    boolean check(Node node, IncrChecker c) {
        if (node == null) return true;
        return check(node.left, c) && c.isValid(node.data) && check(node.right, c);
    }

    boolean checkBST(Node root) {
        IncrChecker checker = new IncrChecker();
        return check(root, checker);
    }