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:

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

Example 2:

    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`