While I agree that this is an example of a particularly ridiculous interview problem, and "gotcha" type interview problems are bad in general, I don't think it's accurate to say it was an open problem for 12 years. An open problem is one that has been posed but unsolved. In this case, the invention of the linked-list data structure in 1955 does not mean anyone had actually posed the problem of cycle-finding. It's possible, even probable, that the problem hadn't been seriously considered until Floyd did so and came up with the algorithm he published in 1967.