Queue2

#include <iostream>

using namespace std;

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

void addValues(int n, unsigned int& counter)
{
    Queue* p = last;

    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(int n, int x, unsigned int& counter)
{
    Queue* p = first;

    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)
        {
            Queue* temp = p->next;
            p->next = temp->next;
            delete temp;
        }
        else
        {
            Queue* temp = p->next;
            p->next = NULL;
            delete temp;
        }

        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");
            int n;
            cout << "Enter a value: ";
            cin >> n;
            addValues(n, counter);
            break;
        }
        case 2:
        {
            system("cls");
            if (first)
            {
                removeValues();
            }
            else
            {
                cout << "The queue is empty" << endl;
            }
            break;
        }
        case 3:
        {
            system("cls");
            if (first)
            {
                printValues();
            }
            else
            {
                cout << "The queue is empty" << endl;
            }
            break;
        }
        case 4:
        {
            system("cls");
            if (first)
            {
                int n, x;
                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;

                removeNAndChange(n, x, counter);
            }
            else
            {
                cout << "The queue is empty" << endl;
            }
            break;
        }
        default:
            break;
        }

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

    return 0;
}