Insert comma on every three digits

gistfile1.java

  private String insertComma(String digits) {
    String result = digits;
    if (digits.length() <= 3) {
      return digits;
    } else {
      for (int i = 0; i < (digits.length() - 1) / 3; i++) {
        int commaPos = (digits.length() - 3) - (3 * i);
        result = result.substring(0, commaPos) + "," + result.substring(commaPos);
      }
      return result;
    }
  }