1.2 由两个栈组成的队列

#include <bits/stdc++.h>
using namespace std;

class myQueue{
private:
    stack<int> s1, s2;

public:
    void add(int x){
        s1.push(x);
    }
    
    void poll(){
        if(s2.empty()){
            while(s1.empty() == false){
                s2.push(s1.top());
                s1.pop();
            }
        }
        s2.pop();
    }
    
    int peek(){
        if(s2.empty()){
            while(s1.empty() == false){
                s2.push(s1.top());
                s1.pop();
            }
        }
        return s2.top();
    }
};

int main(){
    int n;
    cin>>n;
    myQueue mq;
    while(n>0){
        --n;
        string op="";
        cin>>op;
        if(op == "add"){
            int temp;
            cin>>temp;
            mq.add(temp);
        }
        else if(op == "poll"){
            mq.poll();
        }
        else
            cout<<mq.peek()<<endl;
    }
}

两个栈，一个做队尾进队，一个做队头出队。只有当队头栈没有元素时，才把队尾栈元素装过来