Queue

#include <iostream>

using namespace std;

struct Queue
{
    int value;
    Queue* next;
} *first = NULL, * last = NULL;



void addValues(unsigned int& counter)
{

    Queue* p = last;

    int n;

    cout << "Enter a value: ";
    cin >> n;

    last = new Queue;

    last->value = n;
    last->next = NULL;

    if (first == NULL)
    {
        first = last;
    }
    else
    {
        p->next = last;
    }

    counter++;

}

void removeValues()
{

    while (first)
    {
        Queue* p = first;

        first = p->next;
        delete p;
    }

    cout << "The queue is already empty" << endl;
    cout << endl;

}

void printValues()
{
    Queue* p = first;

    while (p != NULL)
    {
        cout << p->value << " ";
        p = p->next;
    }
    cout << endl;
}

void removeNAndChange(unsigned int& counter)
{
    int n;
    int x;

    Queue* p = first;

    do
    {
        cout << "Enter a number of element to remove" << "[" << counter << "-2]: ";
        cin >> n;

    } while (n < 2 || n > counter);

    cout << "Enter a new value: ";
    cin >> x;

    if (n == 2)
    {
        p = p->next;

        first->value = x;

        first->next = p->next;

        delete p;

        counter--;

    }
    else {

        for (int i = 2; i <= n - 1; i++)
        {
            p = p->next;

        }

        p->value = x;

        if (n < counter)
        {
            p->next = p->next->next;

            p = p->next;

            delete p;
        }
        else
        {
            p->next = NULL;
        }

        counter--;
    }

}

int main()
{
    short choose = -1;

    unsigned int counter = 0;

    do
    {

        cout << "[1] Add a value to the queue\n[2] Remove element/s\n[3] Print all elements\n[4] Remove N element by last to start and Change value on N-1\n";
        cin >> choose;

        switch (choose)
        {

        case 1:

            system("cls");

            addValues(counter);


            choose = -1;


            break;

        case 2:

            system("cls");

            if (first)
            {
                removeValues();
            }
            else
            {
                cout << "The queue is empty" << endl;
            }

            choose = -1;

            break;


        case 3:

            system("cls");

            if (first)
            {
                printValues();
            }
            else
            {
                cout << "The queue is empty" << endl;
            }

            choose = -1;

            break;
        case 4:

            system("cls");

            if (first)
            {
                removeNAndChange(counter);
            }
            else {
                cout << "The queue is empty" << endl;
            }

            choose = -1;

            break;
        default:
            break;
        }

    } while (choose < 0 || choose > 4);
}