Given the root of a binary tree, return true if it is a valid binary search tree, otherwise return false.

A valid binary search tree satisfies the following constraints:

• The left subtree of every node contains only nodes with keys less than the node's key.
• The right subtree of every node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees are also binary search trees.

Example 1:

Input: root = [2,1,3]

Output: true

Example 2:

Input: root = [1,2,3]

Output: false

Explanation: The root node's value is 1 but its left child's value is 2 which is greater than 1.

Constraints:

• 1 <= The number of nodes in the tree <= 10000.
• -1000000000 <= Node.val <= 1000000000