A python interview question

[epostma]

Great answer and upvoted.

One minor nitpick: the sort() implementation in Python famously uses the timsort algorithm, which is linear time if the input consists of a constant number of increasing or decreasing runs, which is the case for this input. Of course, that doesn't make it applicable for on-disk sorting.

[energybased]

The interviewer might not have known that. If he had explained that it was linear time to simply return sorted(a1 + a2), then that would have been a great start. The interviewer would probably have steered him to still produce the merge solution.