void swap(Node** head, int index1, int index2)
{
    if (*head == nullptr)
        return;

    Node *a = nullptr, *b = nullptr, *t = *head;

    for (int i = 0; i <= std::max(index1, index2); ++i)
    {
        if (i == index1)
            a = t;

        if (i == index2)
            b = t;

        t = t->next;
    }

    Node *a_prev = a->prev, *a_next = a->next,
         *b_prev = b->prev, *b_next = b->next;

    if (a_next != nullptr)
        a_next->prev = b;
    if (a_prev != nullptr)
        a_prev->next = b;

    if (b_next != nullptr)
        b_next->prev = a;
    if (b_prev != nullptr)
        b_prev->next = a;

    a->next = b_next;
    a->prev = b_prev;

    b->next = a_next;
    b->prev = a_prev;

    if (index1 == 0)
        *head = b;

    if (index2 == 0)
        *head = a;
}