public static String rand_int_string(int len) {
        Random r = new Random();
        StringBuilder ret = new StringBuilder();
        if (len == 0) {
            return "0";
        }
        if (r.nextBoolean()) {
            ret.append('-');
        }
        ret.append((char)('1' + r.nextInt(9)));
        while (--len != 0) {
            ret.append((char)('0' + r.nextInt(10)));
        }
        return ret.toString();
    }