I got the thumbs up on this both times. Be sure to use a language you are very proficient in. You have very limited time so just solve the problem provided, as the interviewer will extend the problem. Ask questions about anything you're not sure on in terms of the scope of the question. Explain along the way your decisions and what you might do better given more time. Talk to the interviewer about obvious weaknesses in your throw away whiteboard code (not checking input parameters for example)