It's not a test of your ability to recollect something. It's a conversation. If you're just regurgitating the greatest code solution ever but can't explain why it's so damned good then you're not very impressive.

There ARE contrived problems that you can just regurgitate an answer for. (Why are manholes covers round? Because manholes are round duh!) And if anyone asks you one from the lists that are on the interwebs, they're probably just asking because that's What Google Does