2.用两个顺序存储结构的栈实现一个队列

       设计一个队列，要求用两个顺序存储结构的栈，通过这两个栈的相关运算实现队列的以下功能:

public:
Queue();//构造函数
bool IsEmpty(); //返回队列是否已空
bool IsFull();//返回队列是否已满
int Serve(string& item);//出队列，赋值于item，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

代码如下：

#include<iostream>
#include<string>
using namespace std;
const int maxstack = 10; 
class Stack
{
public:
   Stack();
   bool empty() const;
   string pop();
   string gettop();
   void push(const string &item);
private:
 friend class Queue;
 int count;
    string entry[maxstack];
};
void Stack::push(const string &item)
{
 if(count>=maxstack)
  cout<<"overflow "<<endl;
 else
  entry[count++]=item;
}
string Stack::pop()
{
 string item;
  if(count==0)
  { 
   item="underflow";
      return item;
  }
  else
  {
   item=entry[count-1];
      --count;
      return item;
  }
}
string Stack::gettop()
{
 string item;
  if(count==0)
  {
   item="underflow";
  
  }
  else
  {
   item=entry[count-1];     
  }
  return item;
 }
bool Stack::empty() const
{
  bool outcome=true;
  if(count>0)
  outcome=false;
  return outcome;
}
Stack::Stack()
{
count=0;
}
class Queue
{
   public:
      Queue();//构造函数
      bool IsEmpty(); //返回队列是否已空
      bool IsFull();//返回队列是否已满
      int Serve(string& item);//出队列，赋值于item，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
      int Print();

 private:
    mutable Stack a;
    mutable Stack b;
};
Queue::Queue()
{

}
bool Queue::IsEmpty()
{
    if((a.empty()==true)&&(b.empty()==true))
        return true;
 else
  return false;
}
bool Queue::IsFull()
{  
 if(a.count>=maxstack)
  {
    return true;
  }
 else
  return false;
}
int Queue::Append(const string &item)
{
 if(IsFull()==true)
  return -1;
     a.push(item);
        return 0;
}
int Queue::Serve(string &item)
{ 
 
   if(a.empty() == true)
            {
                item ="/0";
                return -1;
            }
            else
            {
                while (a.empty() == false)
                {
                    b.push(a.pop());
                }

                item = b.pop();

                while (b.empty() == false)
                {
                    a.push(b.pop());
                }

                return 0;
            }
}
int Queue::Retrieve(string &item) const
{
          if (a.empty() == true)
            {
                item ="/0";
                return -1;
            }
            else
            {
                while (a.empty() == false)
                {
                    b.push(a.pop());
                }

                item = b.gettop();

                while (b.empty() == false)
                {
                   a.push(b.pop());
                }

                return 0;
            }           
}
int Queue::Print()
{
 
           if (a.empty() == true)
            {
                return -1;
            }
            else
            {
                cout<<"当前队列元素为: ";

                while (a.empty() == false)
                {
                    b.push(a.pop());
                }               

                while (b.empty() == false)
                {                 
                    cout<<b.gettop()<<" ";
                    a.push(b.pop());
                }
               
                cout<<endl;

                return 0;
            }

}

int main()
{
   Queue q;
   cout<<"1)入队列：Jan,Feb,Mar,Apr,May"<<endl;

   q.Append("Jan");
   q.Append("Feb");
   q.Append("Mar");
   q.Append("Apr");
   q.Append("May");
   q.Print();
   cout<<endl;

   cout<<"2) 出队列二次 - 即Jan、Feb出队列"<<endl;         
   string temp;
   q.Serve(temp);
   q.Serve(temp);
   q.Print();
   cout<<endl;

   cout<<"3) 入队列Jun、Ocb"<<endl;           
            q.Append("Jun");
            q.Append("Oct");
            q.Print();
   cout<<endl;
  
   cout<<"4) 出队列二次"<<endl;
   string temp2;
   q.Serve(temp2);
   q.Serve(temp);
   q.Print();
   cout<<endl;
  
   cout<<"5) Dec入队列"<<endl;
   q.Append("Dec");
   q.Print();

  return 0;
}