In a LeetCode-style interview, wise to admit to seeing a problem before?

No. Pure leetcode interviews are flawed by design. Personally, I love leetcode and have received offers from FAANG et al, but if they put no effort into the value of their questions, I would just solve it and move on. If you aren’t a good fit and they find that out later, it’s on the company to reevaluate how they interview. Obviously a personal opinion that might be a moral gray area, but it’s time for companies to rethink these pure leetcode styled interviews and ask things with substance.

Recently I had some extremely thought provoking conversations with a team. We discussed architecture design, scalability, optimizations I’ve done in my own work, how I think about architecting systems and we did some pair programming. It was awesome! They evaluated me from so many different angles. Some personality evaluations, interviews where I got to ask questions back to back to them, relevant questions to my experience, hands on pair programming. Sure it was a bit long, but I got to interview THEM as well. The final interview was the manager wanting feedback on the interview itself and if it was fair, so they could pivot and improve if necessary.

Btw this wasn’t even a small company, which goes to show anyone _could_ do these types of interviews if they trained the interviewers properly.

Whether you admit to having seen a problem before is up to you but make damned sure you are able to keep your story straight if you don't. We had a candidate say they hadn't seen one of our warm-up problems before when explicitly asked only to later find out that one of our folks had accidentally used the same question for the phone screen. It's never a point in the candidate's favor if there's suspicion that they lied about something.

Really though, if you think LeetCode style questions aren't worthwhile but the employer does, there's a big cultural mismatch between you and them already. Simply go elsewhere and both of you will be happier in the end.

Sure. If they are prepared, they will throw you a curve just to see how you approach it. If they're not prepared, well, that tells you something too.