Simple Double Stack

#include <iostream>
#include <stdlib.h>
#include <string.h>
//#include <conio.h>
#include <windows.h>

using namespace std;


struct produk{
    char nama[20];
};

int top = -1;
int n;
int top2;
int atas = 0, bawah = 0;
produk stacks[20];

void push(){
    top++;
}

void push2(){
    top2--;
}

void pop(){
    top--;
}

void pop2(){
    top2++;
}

void input(){
    push();
    cin.ignore();
    cout<<"Masukan Nama = ";cin.getline(stacks[top].nama, 20);
    atas++;
}

void input2(){
    push2();
    cin.ignore();
    cout<<"Masukan Nama = ";cin.getline(stacks[top2].nama, 20);
    bawah++;
}

void tampil(){
    for(int i=top; i>=0; i--){
        cout<<"\nNama Pembeli = "<<stacks[i].nama;
        cout<<endl;
    }
    for(int a=top2; a<n; a++){
        cout<<"\nNama Pembeli = "<<stacks[a].nama;
        cout<<endl;
    }
}

bool isFull(){
    int penuh = atas + bawah;
    if(penuh == n){
        return true;
    }
    else{
        return false;
    }
}

bool isEmpty(){
    if(top==-1){
        return true;
    }
    else{
        return false;
    }
}

bool isEmpty2(){
    if(top2==n-1){
        return true;
    }
    else{
        return false;
    }
}

int main()
{
    int menu;
    char ulang;
    cout<<"Masukan Panjang Stacks = ";cin>>n;
    top2 = n;
    system("cls");
    do{
    cout<<"\nMenu";
    cout<<"\n1. Input 1";
    cout<<"\n2. Input 2";
    cout<<"\n3. Delete 1";
    cout<<"\n4. Delete 2";
    cout<<"\n5. View";
    cout<<"\nPilihan anda = ";cin>>menu;
    switch(menu){
    case 1:
        if(!isFull()){
            input();
        }
        else{
            cout<<"Stack is FULL!";
        }
        break;
    case 2:
        if(!isFull()){
            input2();
        }
        else{
            cout<<"Stacks is FULL!";
        }
        break;
    case 3:
        if(!isEmpty()){
            pop();
            atas--;
            cout<<"\nData terhapus";
        }
        else{
            cout<<"\nStacks IS NULL!";
        }
        break;
    case 4:
        if(!isEmpty2()){
            pop2();
            bawah--;
            cout<<"\nData Terhapus ";
        }
        else{
            cout<<"\nStack IS NULL!";
        }
        break;
    case 5:
        system("cls");
        tampil();
        break;

    }
    cout<<"\n\nUlang = ";cin>>ulang;
    system("cls");
    }while(ulang=='y');
    return 0;
}