Source: https://leetcode.com/problems/validate-binary-search-tree/

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 key. The right subtree of a node contains only nodes with keys greater than the node key. Both the left and right subtrees must also be binary search trees.

/**
* Definition for a binary tree node.
* function TreeNode(val, left, right) {
*     this.val = (val===undefined ? 0 : val)
*     this.left = (left===undefined ? null : left)
*     this.right = (right===undefined ? null : right)
* }
*/
/**
* @param {TreeNode} root
* @return {boolean}
*/
var isValidBST = function(root) {
 // Keep track of the min and max that a node can be in
 const isValidBSTHelper = (root, min, max) => {
   // if root is null, it's valid so we return null
   if (root === null) {
     return true;
   }
   
   // if root is not null and there is a min node and the root's val <= min node's val, return false
   if (min !== null && root.val <= min.val) {
     return false;
   }
   // if root is not null and there is a max node and the root's val >= max node's val, return false
   if (max !== null && root.val >= max.val) {
     return false;
   }
   
   // recursively check if the left subtree is valid bst and right subtree is valid bst
   return isValidBSTHelper(root.left, min, root) && isValidBSTHelper(root.right, root, max);
 };
 
 return isValidBSTHelper(root, null, null);
};