pop操作：如果s2不为空，直接弹出s2的数据，否则依次弹出s1的数据到s2中，再取2栈顶的数据.

#include <iostream>
#include <stack>
using namespace std;
template<typename T>
class Queue {
public:
    Queue() : qsize(0) { }
    void push(const int &t) { s1.push(t); ++ qsize; }
    T front();
    void pop();
    size_t size() { return qsize; }
    bool empty() { return qsize == 0; }
private:
    size_t qsize;
    stack<int> s1;
    stack<int> s2;
};
template<typename T>
T Queue<T>::front() {
    if (s2.empty()) {
        if (s1.empty())
            throw;
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
    }
    return s2.top();
}
template<typename T>
void Queue<T>::pop() {
    if (s2.empty()) {
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
    }
    if (!s2.empty()) {
        s2.pop();
    }
    --qsize;
}
int main()
{
    Queue<int> mq;
    for (int i = 0; i < 10; ++i)
        mq.push(i);
    while (!mq.empty()) {
        cout << mq.front() << endl;
        mq.pop();
    }
    return 0;
}