private static void addProblem(Element problem) {
    String fileName = problem.getChildText(FILE_ELEMENT);
    ArrayList<Element> problemList = ourFileToProblem.get(fileName);
    if (problemList == null) {
      problemList = new ArrayList<>();
      ourFileToProblem.put(fileName, problemList);
    }
    problemList.add(problem);
  }
private static void addProblem(Element problem)