I understand what you're saying, but I think this particular trick question is probably too much of a trick. "Write a bubble sort" really comes across as a test of familiarity with algorithms, not a test of lateral thinking skills. I guess if you're looking for a person who will respond to the seemingly straightforward "implement algorithm X" with "why don't I do Y instead?", then you'll probably find someone who responds that way. But ultimately I think you're probably tricking yourself, as you will miss out on talented folks who just happened to take your challenge at face value.