public void checkQuestions() {
        Iterator<ContestUser> iterator = users.iterator();
        while(iterator.hasNext()) {
            ContestUser user = iterator.next();
            HashSet<Integer> used = user.getUsed();
            Iterator<Integer> iterator2 = used.iterator();
            while(iterator2.hasNext()) Server_Loader.questions.remove(iterator2.next());
        }
    }