I think it's both. If all the celestial bodies align and your luck is through the roof, you'll get exact problems you've solved before with the optimal solutio

I think it's both. If all the celestial bodies align and your luck is through the roof, you'll get exact problems you've solved before with the optimal solutions ingrained in your mind. But second best is you'll get problems that are at least similar and you are familiar with the patterns, tricks, and gimmicks to go about tackling it, instead of having to try to think up a solution completely from scratch. Horrible luck? It's a question you've seen and practiced but your brain freezes and you can't remember how you solved it. Or something you've seen and earmarked to practice, but couldn't get to it before the interview day. This has happened to me.