code: public List<String> extractSuffixByWords(int length, int size, boolean extend)
      {
          TFDictionary suffixTreeSet = new TFDictionary();
          for (String key : tfDictionary.keySet())
          {
              List<Term> termList = StandardTokenizer.segment(key);
              if (termList.size() > length)
              {
                  suffixTreeSet.add(combine(termList.subList(termList.size() - length, termList.size())));
                  if (extend)
                  {
                      for (int l = 1; l < length; ++l)
                      {
                          suffixTreeSet.add(combine(termList.subList(termList.size() - l, termList.size())));
                      }
                  }
              }
          }
  
          return extract(suffixTreeSet, size);
      }
docs: 此方法认为后缀一定是整个的词语，所以length是以词语为单位的
  @param length
  @param size
  @param extend
  @return