List.erase

void List::erase(int index){
    if (index == 0){
        pop_front();
    }
    else if (index == (n - 1)){
        pop_back();
    }
    else{
        Node* current = get_node_ptr(index);
        Node* previous = get_node_ptr(index - 1);
        Node* next = get_node_ptr(index + 1);

        next->previous = current->previous;
        previous->next = current->next;
        delete current;
        --n;
    }
}