建议：重构rear，用另一个变量检测rear。
by 于洪铎 @ 2021-07-28 13:27:49
建议用STL队列。
by 庄nnnn额 @ 2021-08-02 11:07:57
用DFS吧，DFS更快
by chenyitian @ 2021-08-08 23:00:46