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.

Example 1:

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

Example 2:

   5
  / \
  1  4
    / \
    3  6
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.