Meh. I used to compete on CS and Math problems and I passed the interview for a lot of big tech companies (senior and staff levels) and I don't think at any of them I could pass the interview if I truly had never seen even a variation of the problem I was given.

They want you to work pretty fast and you only have about 20 minutes to solve one problem at most companies, no way I could work my way through something completely new that quickly.