> We also used atoi/itoa

Do you mean the C function atoi() to convert a string to number? If yes, I also used the same technical interview question for year. It is a nice "fractally complex" (many levels of depth) question. Everyone can understand it in 10 seconds, but you can really show your experience when you get into the edge cases and unit tests. Plus, if the candidate is a bit junior, you can push their boundaries by asking about different edge case and watch how they react. Similarly: I had a teammate for a few years that asked people to program a very simple linked list class. He was shocked how many mid-level devs couldn't do it. As I recall, he never sent me a bad candidate, so it was good enough for us.