void str_reverse(char* str)
{
  static char* head;
  char* tail;
  char tmp;
  if (!head)
    head = str;
  if (*str)
    str_reverse(str + 1);
  if (!head)
    return;
  tail = str - 1;
  if (tail <= head) {
    head = 0;
    return;
  }
  tmp = *head;
  *head = *tail;
  *tail = tmp;
  head++;
}

int main()
{
  char s[] = "hello";
  //char s[] = "hell";
  //char s[] = "";
  str_reverse(s);
}