98. Validate Binary Search Tree #

Problem Statement: #

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

A valid 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: #

Input: root = [2,1,3]
Output: true

Example 2: #

Input: root = [5,1,4,null,null,3,6]
Output: false

Explanation: The root node's value is 5 but its right child's value is 4.

Constraints: #

• The number of nodes in the tree is in the range [1, 104].
• -231 <= Node.val <= 231 - 1

Solution: #

public class ValidateBinarySearchTree {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    private boolean isValidBST(TreeNode root, long min, long max) {
        if (root == null)
            return true;
        return (root.val > min &&
                root.val < max &&
                isValidBST(root.left, min, root.val) &&
                isValidBST(root.right, root.val, max));
    }
}

... #