I’ve had interviewers ask about cycle detection in a linked list. The answer was of course the classic tortoise and hare algorithm. Which took ~15 years to come up with, and was found by a Turing award winner.

Anybody who claimed they “came up with the algorithm themselves” in a few minutes in an interview is a liar, and anyone who takes them at their word is a fool.

Well, it's not a rocket science algorithm. But regardless, it's a very bad interview question indeed.