两个栈实现双端队列

一个笔试题，当时竟然没想出来，现在实现下

/*
用两个栈实现双端队列
栈s1，s2。
pushback()和popback(),必须在s2为空的情况，把s2的都放s1中
pushfront()和popfront(),必须是在s1为空，把s1的都给放到s2中
*/
#include <iostream>
#include <stack>
using namespace std;
template <typename T>
class DequeBy2Stack
{
private:
    int maxsize;
    stack<T> s1;
    stack<T> s2;
public:
    DequeBy2Stack(int size):maxsize(size){}
    void pushback(T n);
    T popback();
    void pushfront(T n);
    T popfront();
    int size();
    bool empty();
};
template <typename T>
bool DequeBy2Stack<T>::empty()
{
    return s2.empty()&&s1.empty();
}
template <typename T>
int DequeBy2Stack<T>::size()
{
    return s1.size()+s2.size();
}
template <typename T>
void DequeBy2Stack<T>::pushback(T n)
{
    if(this->size()==maxsize)
    {
        throw new std::exception("Invalid push");
    }
    while(!s2.empty())           //必须将s2中的数据都导入到s1中
    {
        int tmp=s2.top();
        s1.push(tmp);
        s2.pop();
    }
    s1.push(n);
}
template <typename T>
T DequeBy2Stack<T>::popback()
{
    while(!s2.empty())
    {
        int tmp=s2.top();
        s1.push(tmp);
        s2.pop();
    }
    if(!s1.empty())
    {
        int tmp=s1.top();
        s1.pop();
        return tmp;
    }
    else
    {
        throw new exception("Invalid popback()");
    }
}
template <typename T>
void DequeBy2Stack<T>::pushfront(T n)
{
    if(this->size()==maxsize)
    {
        throw new std::exception("Invalid push");
    }
    while(!s1.empty())
    {
        int tmp=s1.top();
        s2.push(tmp);
        s1.pop();
    }
    s2.push(n);
}
template <typename T>
T DequeBy2Stack<T>::popfront()
{
    while(!s1.empty())
    {
        int tmp=s1.top();
        s2.push(tmp);
        s1.pop();
    }
    if(!s2.empty())
    {
        int tmp=s2.top();
        s2.pop();
        return tmp;
    }
    else
    {
        throw new exception("Invalid popfront");
    }
}

int main()
{
    DequeBy2Stack<int> test(8);
    int n;
    for(int i=1;i<=8;i++)
    {
        test.pushback(i);
    }
    test.popback();
    test.popfront();
    test.popfront();
    test.popfront();
    test.pushfront(100);
    test.pushfront(101);
    test.pushback(30);
    test.pushfront(102);
    test.popfront();
    while(!test.empty())
        cout<<test.popback()<<endl;
    system("pause");
}