Validate BST

Problem

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 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.

Examples

isValidBST([1,null,2]);
// returns true
// The node on the right (2) has a number greater
// than the one on the trunk (1)

isValidBST([10,5,15,null,null,6,20]);
// returns false