struct elem
{
    int value;
    struct elem *next;
    struct elem *prev;
};

struct elem *reverseDoubleLinkedList(struct elem *head)
{
    // a -> b -> c ==> a <- b <- c

    struct elem *newHead = nullptr;
    struct elem *current_elem = head;

    while (current_elem != nullptr)
    {
        newHead = current_elem;

        struct elem *next = current_elem->next;
        current_elem->next = current_elem->prev;
        current_elem->prev = next;

        current_elem = next;
    }

    return newHead;
}