Validate Binary Search Tree

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

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Example 1

    2
   / \
  1   3

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

Example 2

    5
   / \
  1   4
     / \
    3   6

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, which is not greater than 5.

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Constraints

• The number of nodes in the tree is in the range [0, 10^4].
• -2^31 <= Node.val <= 2^31 - 1