So I've always wondered about these leetcode interviews, having never suffered one myself, if you're given one you've seen before, are you still supposed to pretend you're working it out from first principles?
When I started practicing leetcode, I underestimate the amount of work to be ready (they say that you should solve 100 problems, I think it's much higher, but obviously some guys are smarter than others...).
That being said, I'm 50-75% of myself during an interview. I managed to fail at questions that I did several times before.
The experience is also different from one interviewer to the next. You can have a guy who asks an easy question, provides help, and still give very positive feedback because he was happy with the way you communicate and explain your algorithm. Then you can have an interviewer (usually more junior ones) who asks a "trick" question that there's no way of solving if you havent' seen it before.
It was at this point that I realized I have no chance of getting into FAANG when I'm playing low-level tactics like just trying to just solve two medium problems in <40 minutes with the most optimal answer. :( (Or 1 hard problem in 30-35 minutes)
But, I would say the majority had not heard the question before.
This is a younger cohort.
Easy, manholes covers are round for the same reason JavaScript is the most popular language.
Answer: once round manholes became ubiquitous it became extremely difficult to make anything other than round manholes covers.
More frequently than one would expect, the candidate will deny having seen the question before, zip through the solution, and then completely stall-out when they are asked to revise their solution to accommodate a very small alteration is made to the problem parameters.