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))