code: def children(self):
          
          self._make_soup()
          child_topic_tag = self.soup.find('div', class_='child-topic')
          if child_topic_tag is None:
              return []
          elif '共有' not in child_topic_tag.contents[-2].text:
              for topic_tag in child_topic_tag.div.find_all('a'):
                  yield Topic(Zhihu_URL + topic_tag['href'],
                              topic_tag.text.strip(),
                              session=self._session)
          else:
              flag = 'load'
              child = ''
              data = {'_xsrf': self.xsrf}
              params = {
                  'parent': self.id
              }
              while flag == 'load':
                  params['child'] = child
                  res = self._session.post(Topic_Get_Children_Url,
                                           params=params, data=data)
                  j = map(lambda x: x[0], res.json()['msg'][1])
                  *topics, last = j
                  for topic in topics:
                      yield Topic(Zhihu_URL + '/topic/' + topic[2], topic[1],
                                  session=self._session)
                  flag = last[0]
                  child = last[2]
                  if flag == 'topic':
                      yield Topic(Zhihu_URL + '/topic/' + last[2], last[1],
                                  session=self._session)
docs: 获取此话题的子话题
  
          :return: 此话题的子话题， 返回生成器
          :rtype: Topic.Iterable