Create Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST). 

Assume a 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.

https://oj.leetcode.com/problems/validate-binary-search-tree/