I used to be reasonably good at algorithmic stuff in university 20 years ago. Since then I hardly ever practice. It's not that I am bad at them, now, but its just so hit and miss whether an answer will come to me in time, you might as well toss a coin. If I was doing algorithmic stuff every day I have no doubt I would be better. Usually what happens is I get out of an interview and a an answer (or a better answer than I gave) pops into my head. Like I say, might as well toss a coin.