Ironically, you just answered the "difficult" interview question in a few lines of casual commenting.
If you're not allowed to use atoi and itoa, it's still not all that difficult to do in C. Incrementing ASCII characters and handling carries is really trivial.
I'd be kind of worried if I was hiring a C developer who didn't know the basic things you described above. I'm not sure why you think those are esoteric concepts that we shouldn't expect developers to know.
Your last 2 sentences highlight the crux of it, I think - in the first sentence, you say "C developer" and in the second just "developers".
This actually does seem like a great icebreaker-type question for a C developer. I'm less convinced every developer should be able to go into that level of detail if they are doing Javascript or Python or whatever. Ideally they'd be able to at least reason through some of this and demonstrate some understanding of some of this, but I'm not sure it is a great icebreaker question for all developer roles.
I didn’t answer the interview question. The interview question would be to actually implement it. Something that was stated should be trivially done in 15 seconds. It would take me longer than that to explain why it’s not a trivial problem.
If you're not allowed to use atoi and itoa, it's still not all that difficult to do in C. Incrementing ASCII characters and handling carries is really trivial.
I'd be kind of worried if I was hiring a C developer who didn't know the basic things you described above. I'm not sure why you think those are esoteric concepts that we shouldn't expect developers to know.