Depends what you mean by reasonable I suppose. If you just implement it naively, either by growing by 1 or a constant amount each time, it'll be O(n) amortized.

I'd call those potentially reasonable, in that it works and it doesn't break the standard. I probably wouldn't be mad if a student did that impl, but I'd mention it.

I would be surprised if a well-supported, non-toy implementation without strange constraints (eg not for embedded and not trying to optimize solely for size or simplicity or something) did something other than amortized O(1).