@Override
    public E remove(int index) {
        rangeCheck(index);

        Node<E> node = node(index);
        Node<E> prev = node.prev;
        Node<E> next = node.next;
        
        if (prev == null) { // index == 0
            first = next;
        } else {
            prev.next = next;
        }
        
        if (next == null) { // index == size - 1
            last = prev;
        } else {
            next.prev = prev;
        }
        
        size--;
        return node.element;
    }

    @Override
    public void add(int index, E element) {
        rangeCheckForAdd(index);
        
        if (index == 0) {
            Node<E> newFirst = new Node<>(element, first);
            // 拿到最后一个节点
            Node<E> last = (size == 0) ? newFirst : node(size - 1);
            last.next = newFirst;
            first = newFirst;
        } else {
            Node<E> prev = node(index - 1);
            prev.next = new Node<>(element, prev.next);
        }
        size++;
    }

    @Override
    public E remove(int index) {
        rangeCheck(index);
        
        Node<E> node = first;
        if (index == 0) {
            if (size == 1) {
                first = null;
            } else {
                Node<E> last = node(size - 1);
                first = first.next;
                last.next = first;
            }
        } else {
            Node<E> prev = node(index - 1);
            node = prev.next;
            prev.next = node.next;
        }
        size--;
        return node.element;
    }

    @Override
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        // size == 0
        // index == 0
        if (index == size) { // 往最后面添加元素
            Node<E> oldLast = last;
            last = new Node<>(oldLast, element, first);
            if (oldLast == null) { // 这是链表添加的第一个元素
                first = last;
                first.next = first;
                first.prev = first;
            } else {
                oldLast.next = last;
                first.prev = last;
            }
        } else {
            Node<E> next = node(index); 
            Node<E> prev = next.prev; 
            Node<E> node = new Node<>(prev, element, next);
            next.prev = node;
            prev.next = node;
            
            if (next == first) { // index == 0
                first = node;
            }
        }
        
        size++;
    }

@Override
    public E remove(int index) {
        rangeCheck(index);
        return remove(node(index));
    }
    
    private E remove(Node<E> node) {
        if (size == 1) {
            first = null;
            last = null;
        } else {
            Node<E> prev = node.prev;
            Node<E> next = node.next;
            prev.next = next;
            next.prev = prev;
            
            if (node == first) { // index == 0
                first = next;
            }
            
            if (node == last) { // index == size - 1
                last = prev;
            }
        }
        
        size--;
        return node.element;
    }