I got just the opposite impression- the interviewer gave a problem that is different from the normal “sorting problem,” particularly so that the answer wouldn’t

I got just the opposite impression- the interviewer gave a problem that is different from the normal “sorting problem,” particularly so that the answer wouldn’t be any of the algorithms one learned in school. He was presumably hoping that the interviewee would get that, figure that the added constraints allowed a more efficient solution, and try to think through a novel algorithm. If the guy had used a bit of creativity, it wouldn’t matter if he didn’t know radix sort from a hole in the ground. I couldn’t promise that I’d make that leap either, sitting sweaty-palmed in front of a hiring manager, but I see what the interviewer was thinking.