1. I did not know about one of the algorithms but I gave them a vague idea of how it should be done. You don't need to have the right answer always. Even when you don't know any algorithm. They are looking to test your thought process in algorithmic questions. 2. When you don't know a particular answer or have no idea about it, you can simply be honest and tell them you don't know and haven't got a chance to work on it. Trust me being honest about it works wonders and sends a positive vibe to the interviewer.