This problem was posed to me for real in a Microsoft interview in the late 90s. At first I was like "How hard could this be?" but quickly discovered that it was

This problem was posed to me for real in a Microsoft interview in the late 90s. At first I was like "How hard could this be?" but quickly discovered that it wasn't so obvious. If I hadn't already seen the famous circular-list detection method, there's no way I would've gotten it, not during the interview at least.