JavaAlgorithms/DataStructures/Trees/ValidBSTOrNot.java

package DataStructures.Trees;

public class ValidBSTOrNot {

    class Node {
        int data;
        Node left, right;

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

    //Root of the Binary Tree
 
    /* can give min and max value according to your code or
    can write a function to find min and max value of tree. */

    /* returns true if given search tree is binary
     search tree (efficient version) */
    boolean isBST(Node root) {
        return isBSTUtil(root, Integer.MIN_VALUE,
                Integer.MAX_VALUE);
    }

    /* Returns true if the given tree is a BST and its
      values are >= min and <= max. */
    boolean isBSTUtil(Node node, int min, int max) {
        /* an empty tree is BST */
        if (node == null)
            return true;

        /* false if this node violates the min/max constraints */
        if (node.data < min || node.data > max)
            return false;
 
        /* otherwise check the subtrees recursively
        tightening the min/max constraints */
        // Allow only distinct values
        return (isBSTUtil(node.left, min, node.data - 1) &&
                isBSTUtil(node.right, node.data + 1, max));
    }
}