//Exercise 6, call
void removeOcc(List **head, int X){
    List *p = *head;
    List *prev = NULL;
    while(p != NULL){
        if((*head)->data == X){
            *head = (*head)->next;
            free(p);
            p = *head;
        }
        else if (p->data == X){
            prev->next = p->next;
            free(p);
            p = prev->next;
        }
        else{
            prev = p;
            p = p->next;
        }
    }
}
//Exercise 7, call: mergeLists(L1, L2, &L3);
void mergeLists(List* L1, List* L2, List** L3) {
    if (L1 == NULL) *L3 = L2;
    else if (L2 == NULL) *L3 = L1;
    else {
        if (L1->data < L2->data) {
            *L3 = L1;
            L1 = L1->next;
        } else {
            *L3 = L2;
            L2 = L2->next;
        }
        List* tail = *L3;
        while (L1 != NULL && L2 != NULL) {
            if (L1->data < L2->data) {
                tail->next = L1;
                L1 = L1->next;
            } else {
                tail->next = L2;
                L2 = L2->next;
            }
            tail = tail->next;
        }
        if (L1 != NULL) tail->next = L1;
        else tail->next = L2;
    }
}

//Exercise 8, call: splitList(L, &L1, &L2);
void splitList(List* L, List** L1, List** L2) {
    *L1 = NULL;
    *L2 = NULL;

    List* tail1 = NULL;
    List* tail2 = NULL;

    while (L != NULL) {
        if (L->data % 2 != 0) {
            if (*L1 == NULL) {
                *L1 = L;
                tail1 = *L1;
            } else {
                tail1->next = L;
                tail1 = tail1->next;
            }
        } else {
            if (*L2 == NULL) {
                *L2 = L;
                tail2 = *L2;
            } else {
                tail2->next = L;
                tail2 = tail2->next;
            }
        }
        L = L->next;
    }

    if (tail1 != NULL) tail1->next = NULL;
    if (tail2 != NULL) tail2->next = NULL;
}

//Exercise 9, call: removeFirst(&head);
void removeFirst(List** head) {
    if (*head != NULL) {

        List* p = *head;
        if (p->next == *head) {
         free(p);
         *head = NULL;

        } else {

            List* last = *head;
            while (last->next != *head) {last = last->next;}

            last->next = p->next;
            *head = p->next;
            free(p);
        }
    }
}
//Exercise 10, call: append(&head, val);
void append(List** head, int val) {
    List* newNode = (List*)malloc(sizeof(List));
    newNode->data = val;
    newNode->next = NULL;
    newNode->prev = NULL;

    if (*head == NULL) {
        *head = newNode;
    } else {
        List* p = *head;
        while (p->next != NULL) {
            p = p->next;
        }
        p->next = newNode;
        newNode->prev = p;
    }
}