code: public static List<Item> normalizeFrequency(List<Item> itemList)
      {
          for (Item item : itemList)
          {
              ArrayList<Map.Entry<String, Integer>> entryArray = new ArrayList<Map.Entry<String, Integer>>(item.labelMap.entrySet());
              Collections.sort(entryArray, new Comparator<Map.Entry<String, Integer>>()
              {
                  @Override
                  public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2)
                  {
                      return o1.getValue().compareTo(o2.getValue());
                  }
              });
              int index = 1;
              for (Map.Entry<String, Integer> pair : entryArray)
              {
                  item.labelMap.put(pair.getKey(), index);
                  ++index;
              }
          }
          return itemList;
      }
docs: 调整频次，按排序后的次序给定频次
  
  @param itemList
  @return 处理后的列表