void insert(int key, int data)
{
    struct item *new, *prev;

    /* create the new item */
    new = (struct item *)malloc(sizeof(struct item));
    /* (should check for NULL return, but omit for this example) */
    new->key = key;
    new->data = data;

    /* find the node it goes after; NULL if it goes at the front */
    if (head == NULL || head->key >= key) {
        prev = NULL;
    } else {
        for (prev = head;
                prev->next && prev->next->key < key;
                prev = prev->next)
            ;
    }

    /* link it in */
    if (prev == NULL) {
        /* goes at the head of the list */
        new->next = head;
        head = new;
    } else {
        /* goes after 'prev' */
        new->next = prev->next;
        prev->next = new;
    }
}


void delete(int key)
{
    struct item *it, *prev = NULL;

    /* find it */
    for (it = head; it && it->key < key; it = it->next)
        prev = it;

    if (it && it->key == key) {
        if (prev) {
            /* delete an item which is not first */
            prev->next = it->next;
        } else {
            /* delete the head */
            head = it->next;
        }
        free((char *)it);
    } else {
        printf("error: %d not found\n", key);
    }
}