def test_to_sorted_doubly_linked_list(self):
     self.class_under_test.insert(1)
     self.class_under_test.insert(3)
     self.class_under_test.insert(4)
     self.class_under_test.insert(2)
     result = b.to_sorted_doubly_linked_list(self.class_under_test)
     self.assertTrue(result.head.data == 1)
     self.assertTrue(result.head.prev.data == 4)