void LList::insert(int num, int at)
{
    node **pp = &root;
    for (; *pp && at; pp = &(*pp)->next, --at);

    if (at == 0)
    {
        node *t = new node(num);
        t->next = *pp;
        *pp = t;
    }
    else
    {
        std::cerr << "invalid index\n";
    }
}