def test_invalid_bst(self):
root = Node(4)
root.left = Node(2)
root.right = Node(5)
root.left.left = Node(3)
self.assertFalse(is_valid_bst(root))
def test_is_valid_bst(self):
bst = BST([1, 2, 3, 4, 5, 6, 7, 8, 9])
self.assertTrue(is_valid_bst(bst.root))