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 values less than the node's value.
• The right subtree of a node contains only nodes with values greater than the node's value.
• Both the left and right subtrees must also be valid BSTs.

The tree is represented as a nested object: { val: 2, left: { val: 1, left: null, right: null }, right: { val: 3, left: null, right: null } }

Example 1:

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

Example 2:

Input: root = [5,1,4,null,null,3,6]
    5
   / \
  1   4
     / \
    3   6
Output: false
Explanation: The root's right child is 4, which is less than 5.