3-5.cpp

#include <iostream>
#include <stack>
using namespace std;
class MyQueue 
{
private:
    stack<int> in;
    stack<int> out;
public:
    void push(int v) {
        in.push(v);
    }
    void pop() {
        if (out.empty()) {
            while (!in.empty()) {
                out.push(in.top());
                in.pop();
            }
        }
        out.pop();
    }
    int front() {
        if (out.empty()) {
            while (!in.empty()) {
                out.push(in.top());
                in.pop();
            }
        }
        return out.top();
    }
};
int main(int argc, char *argv[]) {
    MyQueue mq;
    mq.push(1);
    mq.push(2);
    mq.push(3);
    cout << "front(): " << mq.front() << endl;
    mq.pop();
    cout << "poped." << endl;
    cout << "front(): " << mq.front() << endl;
    mq.pop();
    cout << "poped." << endl;
    cout << "front(): " << mq.front() << endl;
    mq.pop();
    cout << "poped." << endl;
    cout << "it should be empty." << endl;
    
    return 0;
}