DLL

typedef struct dl_list_t dl_list_t;
struct dl_list_t
{
    char   *str; //yes, this is just a string since you're porting to java

    dl_list_t  *next, *prev;
};

dl_list_t *dl_list_create()
{
    dl_list_t *tmp = malloc(sizeof(dl_list_t));
    if(tmp == NULL)
        return NULL;

    memset(tmp, 0, sizeof(dl_list_t));
    return tmp;
}

dl_list_t *dl_list_free(dl_list_t *head)
{
    while(head->prev != NULL)
        head=head->prev;
    while(head != NULL)
    {
        dl_list_t *tmp = head;
        head=head->next;
        free(tmp->str);
        free(tmp);

    }
    return head;
}

dl_list_t *dl_list_insert(dl_list_t *dl_list, char *stri)
{
    if(dl_list == NULL)
        return NULL;
    dl_list_t *tmp = dl_list;

    if(tmp->next != NULL)
        tmp=tmp->next;

    if(tmp->str == NULL)
    {
        tmp->str = malloc(strlen(cats)+1);
        strcpy(tmp->str, cats);
    }
    else if(strcmp(tmp->str, cats) != 0)
    {
        tmp->next = dl_list_create();
        if(tmp->next == NULL)
            return NULL;
        tmp->next->prev = tmp;
        tmp = tmp->next;
        tmp->str = malloc(strlen(cats)+1);
        strcpy(tmp->str, cats);
    }

    return tmp;
}