I'm sure many interviewers do cargo-cult whiteboard interviews, but the core intention of the archetypical whiteboard interview is to spring a complex question on the candidate that they have never seen before and observe how the candidate thinks. If the interviewer is asking questions that the candidate knows the answer to the interviewer is literally doing it wrong. It's not a trivia challenge, and the interviewer should only ask questions that they are confident the candidate hasn't ever seen before. That's not to say that Leetcode-style preparation doesn't work, but it works by tricking the interviewer into thinking that the candidate has solved the problem very quickly, when in fact the candidate actually already knew the solution.
Once you "memorize" a core set of data structures and algorithms, you can apply them to a variety of "novel" problems. The issue is those technique that are memorized, that become useful in solving these problems, aren't intuitive, despite their deceptive simplicity. In reality there's little memorization of solutions, and much memorization of useful techniques (data structures, algorithmic techniques) that are useful for these problems. And the more you do, the more you'll have at your disposal for "novel" problems. Further, many of these memorized techniques are not as intuitive as they seem. Quite a few resulted in publications when they were discovered.
5 years ago the fast/slow pointer technique for cycle detection could have been impossibly hard, now I see questions that just assume the candidate knows this. Every year the bar gets higher.
