用双头栈模拟队列，要求实现一个队列，提供Enqueue(&q, e)和Dequeue(&q, &e)两个接口

保证先进先出，用双头栈DStack实现（分A、B两个头），DStack只提供pushA(), popA(), pushB(), popB()等几个接口，不能访问任何位置i的元素，不能进行任意位置的插入、删除操作-Queue with a double stack simulation, called for a queue to provide Enqueue (& q, e) and Dequeue (& q, & e) two interfaces

Ensure FIFO, with the double stack DStack implementation (points A, B two heads), DStack only pushA (), popA (), pushB (), popB (), and several other interfaces, you can not access any location of the element i , can not be inserted at any position, delete