root of a binary tree, determine if it is a valid binary search tree (BST).
A valid BST is defined as follows:
Input: root = [2,1,3] Output: true
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.
-231 <= Node.val <= 231 - 1