用单链表类来表示一个双端队列，要求可在表的两端插入（链表头和尾，

即可用向前和也可用向后生成），但只能在表的一端删除（链表头）。给出基于

此结构队列的两个入队和一个出队算法的成员函数。-A single linked list class to represent a double-ended queue, requirements can be inserted into the ends of the table (list head and tail can also be used with forward and backward generation), but only one end of the table delete (list head). Given queue based on this structure into two teams and a member of the team algorithm functions.