code: def train(self, word_s):
          
          for word in word_s:
              # 把词语的每个前缀更新到 prefix_set 中
              for index in range(len(word)):
                  self._set.add(word[:index + 1])
docs: 更新 prefix set
  
          :param word_s: 词语库列表
          :type word_s: iterable
          :return: None