java双向链表的认识与实现

java双向链表

双向链表也叫双链表，是链表的一种，它的每个数据结点中都有两个指针，分别指向直接后继和直接前驱。所以，从双向链表中的任意一个结点开始，都可以很方便地访问它的前驱结点和后继结点。

代码实现

public class MyLinkedList<E> {
    int size = 0;
    Node<E> first;
    Node<E> last;

    private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

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

    // 头部新增
    void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
    }

    // 尾部新增
    void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
    }

    // 指定元素前新增
    void linkBefore(E e, Node<E> succ) {
        final Node<E> prev = succ.prev;
        final Node<E> newNode = new Node<>(prev, e, succ);
        succ.prev = newNode;
        if (prev == null)
            first = null;
        else
            prev.next = succ;
        size++;
    }

    // 头部删除
    E unLinkFirst(Node<E> f) {
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null;
        first = next;
        if (next == null)
            first = null;
        else
            next.prev = null;
        size--;
        return element;
    }

    // 尾部删除
    E unLinkLast(Node<E> l) {
        final E element = l.item;
        final Node<E> prev = l.prev;
        l.item = null;
        l.prev = null;
        last = prev;
        if (prev == null)
            last = null;
        else
            prev.next = null;
        size--;
        return element;
    }

    // 指定元素删除
    E unLink(Node<E> x) {
        final E element = x.item;
        final Node<E> prev = x.prev;
        final Node<E> next = x.next;
        if (prev == null)
            first = next;
        else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null)
            last = prev;
        else {
            next.prev = prev;
            x.next = null;
        }
        x.item = null;
        size--;
        return element;
    }

    // 获取链表第一个元素
    public E getFirst() {
        final Node<E> f = first;
        if (f == null) {
            throw new NoSuchElementException();
        }
        return f.item;
    }

    // 获取链表最后一个元素
    public E getLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return l.item;
    }

    // 移除链表第一个元素
    public E removeFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return unLinkFirst(f);
    }

    // 移除链表最后一个元素
    public E removeLast() {
        final Node<E> l = last;
        if (l == null)
            throw new NoSuchElementException();
        return unLinkLast(l);
    }

    // 在列表的开头插入指定的元素
    public void addFirst(E e) {
        linkFirst(e);
    }

    // 将指定的元素追加到列表的末尾
    public void addLast(E e) {
        linkLast(e);
    }

    // 判断列表中是否有该元素
    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    // 返回列表中元素的个数
    public int size() {
        return size;
    }

    // 将指定元素追加到链表的末尾
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    // 将指定元素移除列表
    public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unLink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unLink(x);
                    return true;
                }
            }
        }
        return false;
    }

    // 从指定位置开始,将指定集合中的全部元素插入到此列表
    public boolean addAll(int index, Collection<? extends E> c) {
        checkPositionIndex(index);

        Object[] arr = c.toArray();
        int len = arr.length;
        if (len == 0)
            return false;

        Node<E> prev, succ;
        if (index == size) {
            succ = null;
            prev = last;
        } else {
            succ = node(index);
            prev = succ.prev;
        }

        for (Object o : arr) {
            @SuppressWarnings("unchecked") E e = (E) o;
            Node<E> newNode = new Node<>(prev, e, null);
            if (prev == null)
                first = newNode;
            else
                prev.next = newNode;
            prev = newNode;
        }
        if (succ == null) {
            last = prev;
        } else {
            prev.next = succ;
            succ.prev = prev;
        }
        size += len;
        return true;
    }

    // 清空链表
    public void clear() {
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.prev = null;
            x.next = null;
            x = next;
        }
        first = last = null;
        size = 0;
    }

    // 返回位于列表中指定位置的元素
    public E get(int index) {
        checkPositionIndex(index);
        return node(index).item;
    }

    // 将列表中指定位置的元素替换为指定元素
    public E set(int index, E element) {
        checkPositionIndex(index);
        Node<E> x = node(index);
        E oldVal = x.item;
        x.item = element;
        return oldVal;
    }

    // 将指定元素插入到列表中的指定位置
    public void add(int index, E element) {
        checkPositionIndex(index);
        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    // 删除位于列表中指定位置的元素
    public E remove(int index) {
        checkPositionIndex(index);
        return unLink(node(index));
    }

    // 返回列表中指定元素第一次出现的索引，如果列表中不包含该元素，则返回-1
    public int indexOf(Object o) {
        int index = 0;
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null)
                    return index;
                index++;
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;
    }

    // 返回列表中指定元素最后一次出现的索引，如果列表中不包含该元素，则返回-1
    public int lastIndexOf(Object o) {
        int index = size;
        if (o == null) {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (x.item == null) {
                    return index;
                }
            }
        } else {
            for (Node<E> x = last; x != null; x = x.prev) {
                index--;
                if (o.equals(x.item)) {
                    return index;
                }
            }
        }
        return -1;
    }

    // 返回指定元素索引处的Node(非空)
    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;
        }
    }

    // 审查索引是否越界
    private void checkPositionIndex(int index) {
        if (!isPositionIndex(index))
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    // 判断索引是否有效
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    // 索引越界异常详细信息
    private String outOfBoundsMsg(int index) {
        return "Index: " + index + ", Size: " + size;
    }

    // 返回一个数组，按正确的顺序(从第一个元素到最后一个元素)包含这个列表中的所有元素
    public Object[] toArray() {
        Object[] result = new Object[size];
        int i = 0;
        for (Node<E> x = first; x != null; x = x.next)
            result[i++] = x.item;
        return result;
    }
}