LeetCode: Validate Binary Search Tree

LeetCode: Validate Binary Search 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.

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public class ValidateBinarySearchTree {
    public boolean isValidBST(TreeNode root) {
        if (root == null || (root.left == null && root.right == null)) {
            return true;
        }
        return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    private boolean helper(TreeNode root, long min, long max) {
        if (root == null) {
            return true;
        }
        if (root.val <= min || root.val >= max) {
            return false;
        }
        return helper(root.right, root.val, max) && helper(root.left, min, root.val);
    }
}