Validate Binary Search Tree

Validate Binary Search Tree tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node’s key. The right subtree of a node contains only nodes with keys greater than the node’s key. Both the left and right subtrees must also be binary search trees.

Example 1:
     2
    / \
   1   3
Binary tree [2,1,3], return true.
Example 2:
     1
    / \
   2   3
Binary tree [1,2,3], return false.

Validate Binary Search Tree Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class ValidateBinarySearchTree {
    public boolean isValidBST(TreeNode root) {
        if(root == null) {
            return true;
        }

        return validBSTRecursive(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean validBSTRecursive(TreeNode root, long minValue, long maxValue) {
        if(root == null) {
            return true;
        } else if(root.val >= maxValue || root.val <= minValue) {
            return false;
        } else {
            return validBSTRecursive(root.left, minValue, root.val) && validBSTRecursive(root.right, root.val, maxValue);
        }
    }
}

Last modified October 4, 2020