24.3_queue_using_stack.cpp

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

//approach 1 using two stacks
/*
class que{
    stack<int>st1;
    stack<int>st2;
    public:
       void push(int x){
            st1.push(x);
        }

    int pop(){
        if(st1.empty() && st2.empty()){
            cout<<"Queue is empty"<<endl;
            return -1;
        }

        if(st2.empty()){
        while(!st1.empty()){
            st2.push(st1.top());
            st1.pop();
         }
        }
        int topval=st2.top();
        st2.pop(); 
        return topval;

    }
    bool empty(){
        if(st1.empty() && st2.empty()){
            return true;
        }
        return false;
    }
};
*/
//approach 2 using inbuilt stack and 1 extra stack by declaring it
class que{
    stack<int>st1;
    public:
       void push(int x){
            st1.push(x);
        }

    int pop(){
        if(st1.empty()){
            cout<<"Queue is empty"<<endl;
            return -1;
        }

        int x=st1.top();
        st1.pop();
        if(st1.empty()){
           return x;
        }
        int item=pop();
        st1.push(x);
        return item;
    }
    bool empty(){
        if(st1.empty()){
            return true;
        }
        return false;
    }
};

int main(){
    que q;
    q.push(1);
    q.push(2);
    q.push(3);
    q.push(4);
    cout<<q.pop()<<endl;
    q.push(5);
    
    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
    return 0;

}