def test_k_more_than_nodes(self):
     self.assertEqual(kth_smallest(self.root, 6), -1)
 def test_fifth_smallest_value(self):    # largest value
     self.assertEqual(kth_smallest(self.root, 5), 20)
 def test_fourth_smallest_value(self):   # second largest value
     self.assertEqual(kth_smallest(self.root, 4), 18)
 def test_third_smallest_value(self):
     self.assertEqual(kth_smallest(self.root, 3), 17)
 def test_second_smallest_value(self):
     self.assertEqual(kth_smallest(self.root, 2), 15)
 def test_smallest_value(self):
     self.assertEqual(kth_smallest(self.root, 1), 9)