You’re given `99999`. Increment it. A naive approach would turn this into `9999:`. Making this work is a bit complicated and involves memory reallocation if you’re in a C-like language and you need to increase the length of the string in order to increment it. You’ll probably want a system where the caller passes a buffer of sufficient size to use for rewriting the string, in case you overflow. Make sure to have a way for the caller to pass the buffer length. You don’t want to allocate because that’s not the way it should be done in C, callers should generally allocate. You could use atoi, increment, and itoa to go back again, but that’s probably “cheating” from the perspective of a leetcode advocate, they likely want you to do this without the stdlib conversion functions.
There’s a bazillion reasons why this sucks as an interview question. There’s no way a correct answer is just something you’ll “get in 15 seconds”.