Two stacks in same array

twin_stacks.cpp

#include <iostream>

using namespace std;

const int N = 10;


class twinStack
{
    int arr[10];

    int firstStackTop = -1;
    int secondStackTop = 10;

public:

    bool twinFull()
    {
        if(firstStackTop<9 && secondStackTop>=0)
        {
            if(secondStackTop > firstStackTop && (secondStackTop - firstStackTop) > 1)
            {
                return false;
            }
            else if(firstStackTop  > secondStackTop && (firstStackTop - secondStackTop) > 1)
            {
                return false;
            }
            else return true;
        }
        else return true;
    }

    bool isEmptyFirst()
    {
        if(firstStackTop == -1)
        {
            return true;
        }
        else return false;
    }

    bool isEmptySecond()
    {
        if(secondStackTop > 9)
        {
            return true;
        }
        else return false;
    }


    void pushFirst(int value)
    {

        if(twinFull() == false)
        {
            firstStackTop++;
            arr[firstStackTop] = value;
        }
        else cout<<"Stack is full, OVERFLOW!"<<endl;
    }


    void pushSecond(int value)
    {

        if(twinFull() == false)
        {
            secondStackTop--;
            arr[secondStackTop] = value;
        }
        else cout<<"Stack is full, OVERFLOW!"<<endl;
    }

    void popFirst()
    {

        if(isEmptyFirst() == false)
        {
            firstStackTop--;
        }
        else cout<<"Stack is Empty, UNDERFLOW!"<<endl;
    }

    void popSecond()
    {

        if(isEmptySecond() == false)
        {
            secondStackTop++;
        }
        else cout<<"Stack is Empty, UNDERFLOW!"<<endl;
    }

    int topFirst()
    {
        if(isEmptyFirst() != true)
        {
            return arr[firstStackTop];
        }
        else
        {
            cout<<"First Stack is Empty";
            return -1;
        }
    }

    int topSecond()
    {
        if(isEmptySecond() != true)
        {
            return arr[secondStackTop];
        }
        else
        {
            cout<<"Second Stack is Empty";
            return -1;
        }
    }




};

int main()
{
    twinStack test;

    for(int i=0; i<10; i++)
    {
        if(i%2 == 0)
        {
            test.pushFirst(i);
        }
        else test.pushSecond(i);
    }


    cout<<test.topFirst()<<endl;
    test.popFirst();
    cout<<test.topFirst()<<endl;
    test.popFirst();
    cout<<test.topFirst()<<endl;
    test.popFirst();
    cout<<test.topFirst()<<endl;
    test.popFirst();
    cout<<test.topFirst()<<endl;
    test.popFirst();
    cout<<test.topFirst()<<endl;
    test.popFirst();

    cout<<test.topSecond()<<endl;
    test.popSecond();
    cout<<test.topSecond()<<endl;
    test.popSecond();
    cout<<test.topSecond()<<endl;
    test.popSecond();
    cout<<test.topSecond()<<endl;
    test.popSecond();
    cout<<test.topSecond()<<endl;
    test.popSecond();
    cout<<test.topSecond()<<endl;
    test.popSecond();


    return 0;

}