I think it's reasonable to have candidates submit a solution to a problem prior to an interview, but if you require it in order to apply I think you'll find tha

I think it's reasonable to have candidates submit a solution to a problem prior to an interview, but if you require it in order to apply I think you'll find that you don't get very many applicants. In the first case, the candidate has entered the pipeline and has some confidence that the position is still open and that you aren't rejecting him out-of-hand (unqualified, etc).