void pop(Stack *s)
{
      Node* iterator;
      if(s->head == NULL)
      {
            printf("ERROR STACK");
            return;
      }
      iterator = s->head;
      while (iterator->next != s->tail)
      {
            iterator = iterator->next;
      }
      iterator->next = NULL;
      s->tail = iterator;
      return;
}