LinkedList底层原理（双向链表）

public class LinkedList<E> {
    private Node<E> first;
    private Node<E> last;
    private int size;

    private class Node<E> {
        private Node<E> prev;
        private Node<E> next;
        E element;

        public Node(Node<E> prev, E e, Node<E> next) {
            this.prev = prev;
            this.element = e;
            this.next = next;

        }
    }

    /**
     * 添加头节点
     */
    public void addFirst(E e) {
        final Node<E> oldFirst = first;
        final Node<E> newNode = new Node(null, e, oldFirst);
        first = newNode;
        if (first == null) {
            first = newNode;
        } else {
            oldFirst.prev = first;
        }
        size++;
    }

    /**
     * 添加末尾节点
     * 
     * @param e
     */
    public void addLast(E e) {
        final Node<E> oldLast = last;
        final Node<E> newNode = new Node(oldLast, e, null);
        last = newNode;
        if (first == null) {
            first = newNode;
        } else {
            oldLast.next = newNode;
            newNode.prev = oldLast;
        }
        size++;
    }

    /**
     * 添加
     * 
     * @param e
     * @return
     */
    public boolean add(E e) {
        addLast(e);

        return true;
    }

    public void add(int index, E e) {
        check(index);
        if (index == size) {
            addLast(e);
        } else {
            Node<E> succ = node(index);// 当前节点
            Node<E> pred = succ.prev;// 前一个节点
            Node<E> newNode = new Node<>(pred, e, succ);// 确定插入的节点
            if (pred == null) {
                first = newNode;
            } else {
                succ.prev = newNode;// 当前节点的前驱
                pred.next = newNode;// 前一个节点的后继
            }
            size++;
        }

    }

    /**
     * @param index
     *            删除当前元素
     */
    public void remove(int index) {
        Node<E> r = node(index);// 要删除的节点

        Node<E> pred = r.prev;// 前一个节点
        Node<E> next = r.next;// 下一个节点
        if (pred == null) {// 删除的是头节点
            first = next;
            size--;
            return;
        }
        if (next == null) {// 删除的是尾节点
            last = pred;
            size--;
            return;
        }
        // 空出当前节点
        pred.next = next;
        next.prev = pred;
        // r.next = null;
        // r.prev = null;
        r.element = null;
        size--;
    }

    /**
     * @param index
     *            二分法获取当前的节点
     */
    private Node<E> node(int index) {
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++) {
                x = x.next;
            }
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--) {
                x = x.prev;
            }
            return x;
        }
    }

    /**
     * @param index
     *            检查插入索引是否合法
     */
    private void check(int index) {
        if (index < 0 || index > size) {
            System.out.println("插入位置不合法");
            return;
        }
    }

    /**
     * @return 链表的长度
     */
    public int size() {
        return this.size;
    }

    public E get(int index) {
        return node(index).element;
    }

    private class Iterator {
        private Node<E> lastReturn;
        private Node<E> next;// 当前节点指向
        private int cursor;// 游标

        public Iterator(int index) {
            next = (index == size) ? null : node(index);
            cursor = index;
        }

        public boolean hasNext() {
            return cursor != size;
        }

        public E next() {
            int i = cursor;
            lastReturn = next;
            next = next.next;
            cursor = i + 1;
            return lastReturn.element;
        }
    }

    public static void main(String[] args) {
        LinkedList<Integer> list = new LinkedList<>();

        list.add(1);
        list.add(13);
        list.add(5);
        list.add(31);
        list.add(2);
        list.add(5, 4);
        list.remove(0);
        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }
        System.out.println("-------");
        System.out.println(list.size());
        System.out.println("-------");
        for(LinkedList.Iterator i= list.new Iterator(0);i.hasNext();){
            System.out.println(i.next());
        }

    }

}