class PathCalculator {

  public String calculate(String absolute, String r1, String r2) {
    if (absolute == null || r1 == null || r2 == null) {
      throw new IllegalArgumentException("You can't pass null strings as input.");
    }

    String[] absolutePath = absolute.split(DIR_SEPARATOR);
    String[] r1Path = r1.split(DIR_SEPARATOR);
    String[] r2Path = r2.split(DIR_SEPARATOR);
    String[] result = calculatePath(absolutePath, r1Path, r2Path);

    StringBuilder stringBuilder = new StringBuilder(DIR_SEPARATOR);
    for (String dir : result) {
      stringBuilder.append(dir);
      stringBuilder.append("/");
    }
    return stringBuilder.toString();
  }

}