If you don't know your Floyd Warshall from your Tarjan from your Manacher then you'll just fumble around thinking you got close unless you have the remarkable gift of being able to derive these from scratch in a whiteboard situation. Also I cannot stress enough how difficult - and entirely pointless - a whiteboarding interview is.

edit: I didn't mean this to come across pessimistic and critical. I just think it's important to serve a dose of reality regarding the standard expected (and hopefully some pointers towards the kind of algorithms theory you should be not only comfortable with but also able to reproduce on a whiteboard under pressure).