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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:​

ex1

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

Example 2:​

ex2

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​

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

        def valid(node, left, right):
            if not node:
                return True
            if not (node.val<right and node.val>left):
                return False

            return (valid(node.left, left, node.val) and
                    valid(node.right, node.val, right))

        return valid(root, float(-inf), float(inf))