I'm late to this discussion, but you don't have to solve it quickly. These are meant to be practice exercises that teach you, and if you can knock it out fast then you haven't learned much. I did this over a period of several days, working on it an hour or so at a time.

Sure, agile development means working quickly. But that only works for the application of algorithms you already know or those that are trivial to engineer. Any real challenge will take real work, and real work is never fast.