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.

Leetcode Link

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.

Code

Python

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:

        def helper(node, low, high):
            if not node:
                return True
            if not (low < node.val < high):
                return False
            return helper(node.left, low, node.val) and helper(node.right, node.val, high)

        return helper(root, -inf, inf)