I did quite a bit of competitive programming in school, and pretty much all the world-class competitive problems are reduced to well-known algorithms. It's quite hard to come up with something new (not proven to be unsolvable for its constraints). I believe problem setters just try to disguise a known algorithm as much as possible.

Then comes the ability/memorization to actually code it, e.g. if I knew it needs coding red-black tree I wouldn't even start.