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.
checklist Constraints
The number of nodes in the tree is in the range [1, 10^4]. -2^31 <= Node.val <= 2^31 - 1