Binary Search Tree Validation

Given a binary tree root, return whether it's a binary search tree. A binary tree node is a binary search tree if :
------(C++)
1. All nodes on its left subtree are smaller than node.val
2. All nodes on its right subtree are bigger than node.val
3. All nodes hold the these properties.

Constraint :
n ≤ 100,000 where n is the number of nodes in root