定义并实现带头结点的循环双向链表类，除构造和析构函数外，成员函数还有取数据元素个数、插入、删除、取数据元素-definition and take the lead in achieving the cycle node type two-way linked list, in addition to the structural and destructor function, members also function from the number of data elements, insert, delete, extract the data elements
SHOW FULL COLUMNS FROM `jrk_downrecords` [ RunTime:0.001904s ]
SELECT `a`.`aid`,`a`.`title`,`a`.`create_time`,`m`.`username` FROM `jrk_downrecords` `a` INNER JOIN `jrk_member` `m` ON `a`.`uid`=`m`.`id` WHERE `a`.`status` = 1 GROUP BY `a`.`aid` ORDER BY `a`.`create_time` DESC LIMIT 10 [ RunTime:0.130391s ]
SHOW FULL COLUMNS FROM `jrk_tagrecords` [ RunTime:0.001364s ]
SELECT * FROM `jrk_tagrecords` WHERE `status` = 1 ORDER BY `num` DESC LIMIT 20 [ RunTime:0.001968s ]
SHOW FULL COLUMNS FROM `jrk_member` [ RunTime:0.001432s ]
SELECT `id`,`username`,`userhead`,`usertime` FROM `jrk_member` WHERE `status` = 1 ORDER BY `usertime` DESC LIMIT 10 [ RunTime:0.005311s ]
SHOW FULL COLUMNS FROM `jrk_searchrecords` [ RunTime:0.001266s ]
SELECT * FROM `jrk_searchrecords` WHERE `status` = 1 ORDER BY `num` DESC LIMIT 5 [ RunTime:0.008290s ]
SELECT aid,title,count(aid) as c FROM `jrk_downrecords` GROUP BY `aid` ORDER BY `c` DESC LIMIT 10 [ RunTime:0.026904s ]
SHOW FULL COLUMNS FROM `jrk_articles` [ RunTime:0.001436s ]
UPDATE `jrk_articles` SET `hits` = 2 WHERE `id` = 154941 [ RunTime:0.023932s ]
