Convert NSInteger into NSString
Umm… this is a complicated solution that makes no sense.  Why not keep it simple?

NSString *intToString(NSInteger numberToConvert)
{
    return [NSString stringWithFormat:@"%ld", numberToConvert];
}


If, on the other hand, the exercise is to implement your own solution from scratch, then I would consider it cheating to use NSMutableString stringWithFormat:.  Either way, I would consider your solution to be a failed response in an interview situation.