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.
**Note:** For this problem, the tree is represented as an array in level-order (BFS) format, where `null` represents missing nodes.
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, 10^4].
- -2^31 <= Node.val <= 2^31 - 1