static function lang($num)
 {
     if (strlen($num) === 1) {
         return SinaTeachParse::slang($num);
     }
     if (strlen($num) === 2) {
         return SinaTeachParse::slang(substr($num, 0, 1)) . '十' . SinaTeachParse::slang(substr($num, 1, 1));
     }
 }