更新 prefix set

        :param word_s: 词语库列表
        :type word_s: iterable
        :return: None

def train(self, word_s):
        """更新 prefix set

        :param word_s: 词语库列表
        :type word_s: iterable
        :return: None
        """
        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])