Validate Binary Search Tree

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A BST is valid only if:

• 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.

Return 1 is given tree is valid, else return 0.

Class TreeNode:
    val (int)
    left (TreeNode)
    right (TreeNode)

Input Format:

First Parameter - TreeNode root

Output Format:

Return the number.

Example 1:

Input:
    root = [11 7 16 3 9]
Output:
    1

Example 2:

Input:
    root = [1 -4 -2 null null -9 5]
Output:
    0
Explanation:
    The root's node value is 1 but it's right child value is -2.

Constraints:

• The number of nodes in the tree is in the range [1,10⁴]
• -2³¹ <= Node.val <= 2³¹ - 1
• Expected Time Complexity: O(N)
• Expected Space Complexity: O(N)