[leetcode] problem 93 – restore ip addresses

public List<String> (String s) {
    List<String> result = new ArrayList<>();
    restoreIpAddressesHelper(result, new ArrayList<>(), s, 0, 0);
    return result;
}

private void restoreIpAddressesHelper(List<String> result, List<String> ip, String s, int idx, int level) {
    if (level == 4 && idx == s.length()) {
        result.add(ip.get(0) + "." + ip.get(1) + "." + ip.get(2) + "." + ip.get(3));
        return;
    }

    if (level == 4 || idx == s.length())
        return;

    for (int i = 1; i <= 3 && idx + i <= s.length(); i++) {
        String str = s.substring(idx, idx + i);

        if (!isValid(str))
            break;

        ip.add(str);
        restoreIpAddressesHelper(result, ip, s, idx + i, level + 1);
        ip.remove(ip.size() - 1);
    }
}

private boolean isValid(String s) {
    if (s.length() > 1 && s.charAt(0) == '0')
        return false;

    int i = Integer.parseInt(s);
    return i <= 255 && i >= 0;
}