LeetCode: 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.

Solution:

Key points: find a way to pass the value of the grand parent. Got the idea from here.
Can also be solved by in-order traversing the BST and then check whether collected values are in the ascending order.