private String toString(List<Integer> l) {
            StringBuilder builder = new StringBuilder();
            int i = l.size() - 1;
            for (; i >= 0; i--) {
                if (l.get(i) != 0) {
                    break;
                }
            }
            for (; i >= 0; i--) {
                builder.append(l.get(i));
            }
            if (builder.length() == 0) {
                return "0";
            }
            return builder.toString();
        }