code: public void addTerm(String key)
      {
          TermFrequency value = trieSingle.get(key);
          if (value == null)
          {
              value = new TermFrequency(key);
              trieSingle.put(key, value);
          }
          else
          {
              value.increase();
          }
          ++totalTerm;
      }
docs: 统计词频
  
  @param key 增加一个词