用二个顺序存储结构的栈，通过这二个栈的相关运算实现队列的以下功能。



Queue() //构造函数

bool empty() //返回队列是否已空

bool full() //返回队列是否已满



int serve() //出队列，成功返回0，否则返回-1

int append(const string &item) //item进队列，成功返回0，否则返回-1

int retrieve(string &item) const //将队列头的值赋给item，成功返回0，否则返回-1

-Storage structure with two sequential stack, two stack through which the relevant operations to achieve the following functions queue. Queue () // constructor bool empty () // Returns whether the queue is empty bool full () // returned to the queue is full int serve () // the queue, return 0 if successful, otherwise-1 int append (const string &　item) // item into the queue, return 0 if successful, otherwise returns-1 int retrieve (string &　item) const // assign the value of the head of the queue item, return 0 if successful, otherwise-1