code: public static Word combine(List<Word> words){
          if(words==null || words.size() < 2){
              return null;
          }
          String key = "";
          for(Word word : words){
              key += word.getText();
              key += " ";
          }
          key=key.trim();
          String value = GENERIC_TRIE.get(key);
          if(value==null){
              return null;
          }
          return new Word(value);
      }
docs: 将多个词合并成一个，返回null表示不能合并
  @param words
  @return