Even better question: Compute the moving median.

Computing a moving average with samples being pumped through an n-element buffer is easy. Doing so for the median requires more thought. It's also very useful e.g. for removing single-sample noise from an audio track, so it's not a meaningless exercise.

I used to translate classic interview questions into not-spoiled-on-the-internet ones by doing this kind of batch to incremental conversion. The count-the-islands one was fun but hard to fit into a 45minute interview.

Eventually most of those started getting spoiled too lol.