спэшл фо илья

#include <iostream>

using namespace std;

struct list {
    int data;
    list* next;
};

struct st {
    list* up, * down;
    st* next;
};

list* t1, * p1, * t2, * p2;
st* s, * t, * p;

int main() {
    int n=1, a, c;
    cout << "Enter a list item\n->";
    cin >> a;
    if (a == 0)
        cout << "list pust" << endl;
    else {
    /*Ввод элементов списка*/
        while (a)
        {
            t = new st;
            t->up = 0; t->next = 0; t->down = 0;
            /*Заполнение верхнего спика*/
            t1 = new list;
            t1->next = 0;
            t1->data = a;
            t->up = t1;

            if (s == 0) s = t; //голова спискa

            else {
                p->next = t;
                p1->next = t1;
            }
            cin >> a;
            n++;
            if (a != 0)
            {
                /*Заполнение нижнего спика*/
                t2 = new list;
                t2->next = 0;
                t2->data = a;
                t->down = t2;
                if (n != 2) p2->next = t2;
                p = t;
                p2 = t2;
                p1 = t1;
                cin >> a;
            }

        }
    }
    p->next = s; //присваивание последнему листу в среднем списке указатель на голову списка
    /*Ввод управления*/
    cout << "Where to go? up(1), down(2), next(3), the end(0)\n->";
    cin >> c;
    p1 = 0; p2 = 0; p = s; n = 0;
    while (c != 0) {
        switch (c) {
        case 1:
            if (n == 0) {
                if (p->up != 0) {
                    cout << "up\n";
                    p1 = p->up;
                }
                else cout << "Up is empty\n";
                n = 1;
            }
            else cout << " No up\n";
            break;
        case 2:
            if (n == 0)
            {
                if (p->down != 0)
                {
                    cout << "down\n";
                    p2 = p->down;
                }
                else cout << "Down is empty\n";
                n = 2;
            }
            else
                cout << " No Down\n";
            break;
        case 3:
            if (n == 0) {
                if (p != 0) {
                    cout << "next\n";
                    p = p->next;
                }
                else cout << "Next is empty\n";
            }
            if (n == 1) {
                if (p1 != 0) {
                    cout << p1->data << endl;
                    p1 = p1->next;
                }
                else cout << "Next is empty\n";
            }
            if (n == 2) {
                if (p2 != 0) {
                    cout << p2->data << endl;
                    p2 = p2->next;
                }
                else cout << "Next is empty\n";
            }
            break;
        }
        cout << "->";
        cin >> c;
    }
    cout << "The program has finished working" << endl;
    return 0;
}