0098: 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 strictly less than the node's key.
  • The right subtree of a node contains only nodes with keys strictly 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

Code Solution

class Solution:
    def isValidBST(self, root):
        def rec(node, lmax, rmin):
            if not node:
                return True
            if node.val <= lmax or node.val >= rmin:
                return False
            return rec(node.left, lmax, node.val) and rec(node.right, node.val, rmin)

        return rec(root, float('-inf'), float('+inf'))