string(240) "get document error：https://zhidao.baidu.com/search?lm=0&rn=10&pn=0&fr=search&dyTabStr=null&word=%E5%85%B3%E9%94%AE%E8%AF%8D%E5%AF%86%E5%BA%A6%E7%9A%84%E5%AE%9A%E4%B9%89 => Failed to connect to 101.89.159.206 port 38100: Connection refused"
关键词密度的定义
