Validate Binary Search Tree

Medium

Given the root of a binary tree, return true if it is a valid binary search tree: every node in the left subtree is strictly less, every node in the right subtree is strictly greater, and both subtrees are themselves valid BSTs.

Example 1

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

Example 2

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

Constraints

• 1 <= number of nodes <= 10^4
• -2^31 <= Node.val <= 2^31 - 1