Regarding:

A) If you're in a language with a good regular expression library, there is a very fast cheesy way ;)

B) I'm equally mystified, I'm satisfied that my solution was correct, but it took 80 lines and multiple passes through the array. I'm impressed that the top tier polished it off in around ten minutes. There must be a fairly simple way.

C) I agree that this one just depends on how quickly you can figure out an algorithm. It's entirely plausible to me that someone with algorithm competition experience has almost literally seen this problem before.