public void insert(String genre, String title) {
    GenreNode genreToInsert = findGenreNode(genre);
    TitleNode titleToInsert;

    if (genreToInsert == null) {

      // 장르 먼저 만든다.
      genreToInsert = new GenreNode(genre);
      GenreNode temp;
      GenreNode position = findGenrePosition(genre);

      temp = position.getNextGenre();
      position.setNextGenre(genreToInsert);
      genreToInsert.setNextGenre(temp);
    }

    if (genreToInsert.isExistTitle(title) == false) { // 장르가 있는 경우이다. 이때는 타이틀만 넣어주면 된다.
      titleToInsert = new TitleNode(title);
      TitleNode temp;
      TitleNode position2 = genreToInsert.findTitlePosition(title);

      temp = position2.getNextTitle();
      position2.setNextTitle(titleToInsert);
      titleToInsert.setNextTitle(temp);
    }
  }
  public void delete(String genre, String title) {
    GenreNode genreToDelete = findGenreNode(genre);
    if (genreToDelete == null) return;
    TitleNode titleToDelete = genreToDelete.findTitleNode(title);
    if (titleToDelete == null) return;

    TitleNode titlePrev = genreToDelete.findTitlePosition(title);

    titlePrev.setNextTitle(titleToDelete.getNextTitle());

    if (genreToDelete.getTitleHead().getNextTitle() == null) {
      GenreNode genrePrev = findGenrePosition(genre);
      genrePrev.setNextGenre(genreToDelete.getNextGenre());
    }
  }