我们在上期学习了链表的概念与结构。了解到单向链表和单向循环链表，我们在这里继续扩展一下，有单向则就就有双向。所以我们这期学习双向循环链表。乃什么是什么循环链表呢？

概念

图示

此处我们使用LinkedList类来实现双向循环链表

我们用这个双向循环链表可以实现我们的线性表，栈以及队列。就是实现各个接口就可以。让我们以及去分析一下这个LinkedList吧.

解析

代码实现

    private class Node {
        E data;
        Node pre;   //直接前驱
        Node next;  //直接后继

        public Node() {
            this(null, null, null);
        }

        public Node(E data) {
            this(data, null, null);
        }

        public Node(E data, Node pre, Node next) {
            this.data = data;
            this.pre = pre;
            this.next = next;
        }

        @Override
        public String toString() {
            return data.toString();
        }
    }

    private Node head;
    private Node tail;
    private int size;

    public LinkedList() {
        head = null;
        tail = null;
        size = 0;
    }

上面属性定义完后就是实现我们所有接口的方法。我们先实现线性表List接口的方法。

图示

代码实现

    @Override
    public void add(int index, E element) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("add index out of range");
        }
        Node n = new Node(element);
        if (size == 0) {
            head = n;
            tail = n;
            tail.next = head;
            head.pre = tail;
        } else if (index == 0) {
            n.pre = head.pre;
            n.next = head;
            head.pre = n;
            head = n;
            tail.next = head;
        } else if (index == size) {
            n.next = tail.next;
            tail.next = n;
            n.pre = tail;
            tail = n;
            head.pre = tail;
        } else {
            Node p, q;
            if (index <= size / 2) {
                p = head;
                for (int i = 0; i < index - 1; i++) {
                    p = p.next;
                }
                q = p.next;
                p.next = n;
                n.pre = p;
                q.pre = n;
                n.next = q;
            } else {
                p = tail;
                for (int i = size - 1; i > index; i--) {
                    p = p.pre;
                }
                q = p.pre;
                q.next = n;
                n.pre = q;
                n.next = p;
                p.pre = n;
            }
        }
        size++;
    }

添加方法写完后，乃接下来就是删除方法了。

代码如下

 @Override
    public void remove(E element) {
        int index = indexOf(element);
        if (index != -1) {
            remove(index);
        }
    }

    @Override
    public E remove(int index) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("remove index out of range");
        }
        E ret = null;
        Node node;
        if (size == 1) {
            ret = head.data;
            head = null;
            tail = null;
        } else if (index == 0) {
            ret = head.data;
            node = head.next;
            head.next = null;
            node.pre = head.pre;
            head.pre = null;
            head = node;
            tail.next = head;
        } else if (index == size - 1) {
            ret = tail.data;
            node = tail.pre;
            tail.pre = null;
            node.next = tail.next;
            tail.next = null;
            tail = node;
            head.pre = tail;
        } else {
            Node p, q, r;
            if (index <= size / 2) {
                p = head;
                for (int i = 0; i < index - 1; i++) {
                    p = p.next;
                }
                q = p.next;
                r = q.next;
                ret = q.data;
                p.next = r;
                r.pre = p;
                q.next = null;
                q.pre = null;
            } else {
                p = tail;
                for (int i = size - 1; i > index + 1; i--) {
                    p = p.pre;
                }
                q = p.pre;
                r = q.pre;
                ret = q.data;
                r.next = p;
                p.pre = r;
                q.next = null;
                q.pre = null;
            }
        }
        size--;
        return ret;
    }

代码如下

 @Override
    public E get(int index) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("get index out of range");
        }
        if (index == 0) {
            return head.data;
        } else if (index == size - 1) {
            return tail.data;
        } else {
            Node p = head;
            for (int i = 0; i < index; i++) {
                p = p.next;
            }
            return p.data;
        }
    }

    @Override
    public E set(int index, E element) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("set index out of range");
        }
        E ret = null;
        if (index == 0) {
            ret = head.data;
            head.data = element;
        } else if (index == size - 1) {
            ret = tail.data;
            tail.data = element;
        } else {
            Node p = head;
            for (int i = 0; i < index; i++) {
                p = p.next;
            }
            ret = p.data;
            p.data = element;
        }
        return ret;
    }

代码如下

   @Override
    public int size() {
        return size;
    }
    @Override
    public int indexOf(E element) {
        Node p = head;
        int index = 0;
        while (!p.data.equals(element)) {
            p = p.next;
            index++;
            if (p == head) {
        return -1;
    }
}
        return index;
                }
    @Override
    public boolean contains(E element) {
        return indexOf(element) != -1;
    }
    @Override
    public boolean isEmpty() {
        return size == 0 && head == null && tail == null;
    }

    @Override
    public void clear() {
        head = null;
        tail = null;
        size = 0;
    }

代码如下

  @Override
    public void sort(Comparator<E> c) {
        if (c == null) {
            throw new IllegalArgumentException("comparator can not be null");
        }
        //插入排序来做
        if (size == 0 || size == 1) {
            return;
        }
        for (Node nodeA = head.next; nodeA != head; nodeA = nodeA.next) {
            E e = nodeA.data;
            Node nodeB;
            Node nodeC;
            for (nodeB = nodeA, nodeC = nodeB.pre; nodeC != tail && c.compare(nodeC.data, e) > 0; nodeB = nodeB.pre, nodeC = nodeC.pre) {
                nodeB.data = nodeC.data;
            }
            nodeB.data = e;
        }
    }

截取子链表函数，toStrig()以及迭代器方法和单向循环链表是一样的。所以我们就不在这分析了。

对于栈和队列的一些方法我们就通过我们刚写过的方法内部直接调用即可。乃我们就直接上完整代码吧。

完整代码

package P3.链式结构;

import p1.接口.Dequeue;
import p1.接口.List;
import p1.接口.Stack;

import java.util.Comparator;
import java.util.Iterator;

//双向循环链表
public class LinkedList<E> implements List<E>, Dequeue<E>, Stack<E>{

    private class Node {
        E data;
        Node pre;   //直接前驱
        Node next;  //直接后继

        public Node() {
            this(null, null, null);
        }

        public Node(E data) {
            this(data, null, null);
        }

        public Node(E data, Node pre, Node next) {
            this.data = data;
            this.pre = pre;
            this.next = next;
        }

        @Override
        public String toString() {
            return data.toString();
        }
    }

    private Node head;
    private Node tail;
    private int size;

    public LinkedList() {
        head = null;
        tail = null;
        size = 0;
    }

    public LinkedList(E[] arr) {
        if (arr == null) {
            throw new IllegalArgumentException("arr can not be null");
        }
        for (E e : arr) {
            add(e);
        }
    }

    @Override
    public void add(E element) {
        add(size, element);
    }

    @Override
    public void add(int index, E element) {
        if (index < 0 || index > size) {
            throw new IllegalArgumentException("add index out of range");
        }
        Node n = new Node(element);
        if (size == 0) {
            head = n;
            tail = n;
            tail.next = head;
            head.pre = tail;
        } else if (index == 0) {
            n.pre = head.pre;
            n.next = head;
            head.pre = n;
            head = n;
            tail.next = head;
        } else if (index == size) {
            n.next = tail.next;
            tail.next = n;
            n.pre = tail;
            tail = n;
            head.pre = tail;
        } else {
            Node p, q;
            if (index <= size / 2) {
                p = head;
                for (int i = 0; i < index - 1; i++) {
                    p = p.next;
                }
                q = p.next;
                p.next = n;
                n.pre = p;
                q.pre = n;
                n.next = q;
            } else {
                p = tail;
                for (int i = size - 1; i > index; i--) {
                    p = p.pre;
                }
                q = p.pre;
                q.next = n;
                n.pre = q;
                n.next = p;
                p.pre = n;
            }
        }
        size++;
    }

    @Override
    public void remove(E element) {
        int index = indexOf(element);
        if (index != -1) {
            remove(index);
        }
    }

    @Override
    public E remove(int index) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("remove index out of range");
        }
        E ret = null;
        Node node;
        if (size == 1) {
            ret = head.data;
            head = null;
            tail = null;
        } else if (index == 0) {
            ret = head.data;
            node = head.next;
            head.next = null;
            node.pre = head.pre;
            head.pre = null;
            head = node;
            tail.next = head;
        } else if (index == size - 1) {
            ret = tail.data;
            node = tail.pre;
            tail.pre = null;
            node.next = tail.next;
            tail.next = null;
            tail = node;
            head.pre = tail;
        } else {
            Node p, q, r;
            if (index <= size / 2) {
                p = head;
                for (int i = 0; i < index - 1; i++) {
                    p = p.next;
                }
                q = p.next;
                r = q.next;
                ret = q.data;
                p.next = r;
                r.pre = p;
                q.next = null;
                q.pre = null;
            } else {
                p = tail;
                for (int i = size - 1; i > index + 1; i--) {
                    p = p.pre;
                }
                q = p.pre;
                r = q.pre;
                ret = q.data;
                r.next = p;
                p.pre = r;
                q.next = null;
                q.pre = null;
            }
        }
        size--;
        return ret;
    }

    @Override
    public E get(int index) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("get index out of range");
        }
        if (index == 0) {
            return head.data;
        } else if (index == size - 1) {
            return tail.data;
        } else {
            Node p = head;
            for (int i = 0; i < index; i++) {
                p = p.next;
            }
            return p.data;
        }
    }

    @Override
    public E set(int index, E element) {
        if (index < 0 || index >= size) {
            throw new IllegalArgumentException("set index out of range");
        }
        E ret = null;
        if (index == 0) {
            ret = head.data;
            head.data = element;
        } else if (index == size - 1) {
            ret = tail.data;
            tail.data = element;
        } else {
            Node p = head;
            for (int i = 0; i < index; i++) {
                p = p.next;
            }
            ret = p.data;
            p.data = element;
        }
        return ret;
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public int indexOf(E element) {
        Node p = head;
        int index = 0;
        while (!p.data.equals(element)) {
            p = p.next;
            index++;
            if (p == head) {
        return -1;
    }
}
        return index;
                }

    @Override
    public boolean contains(E element) {
        return indexOf(element) != -1;
    }


    @Override
    public boolean isEmpty() {
        return size == 0 && head == null && tail == null;
    }


    @Override
    public void clear() {
        head = null;
        tail = null;
        size = 0;
    }

    @Override
    public void sort(Comparator<E> c) {
        if (c == null) {
            throw new IllegalArgumentException("comparator can not be null");
        }
        //插入排序来做
        if (size == 0 || size == 1) {
            return;
        }
        for (Node nodeA = head.next; nodeA != head; nodeA = nodeA.next) {
            E e = nodeA.data;
            Node nodeB;
            Node nodeC;
            for (nodeB = nodeA, nodeC = nodeB.pre; nodeC != tail && c.compare(nodeC.data, e) > 0; nodeB = nodeB.pre, nodeC = nodeC.pre) {
                nodeB.data = nodeC.data;
            }
            nodeB.data = e;
        }
    }

    @Override
    public List<E> subList(int fromIndex, int toIndex) {
        if (fromIndex < 0 || toIndex >= size || fromIndex > toIndex) {
            throw new IllegalArgumentException("0 <= fromIndex <= toIndex < size");
        }
        Node nodeA = head;
        for (int i = 0; i < fromIndex; i++) {
            nodeA = nodeA.next;
        }
        Node nodeB = head;
        for (int i = 0; i < toIndex; i++) {
            nodeB = nodeB.next;
        }
        Node p = nodeA;
        LinkedList<E> list = new LinkedList<>();
        while (true) {
            list.add(p.data);
            if (p == nodeB) {
                break;
            }
            p = p.next;
        }
        return list;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append('[');
        if (isEmpty()) {
            sb.append(']');
        } else {
            Node p = head;
            while (true) {
                sb.append(p.data);
                if (p == tail) {
                    sb.append(']');
                    break;
                }
                sb.append(',');
                sb.append(' ');
                p = p.next;
            }
        }
        return sb.toString();
    }

    @Override
    public Iterator<E> iterator() {
        return new LinkedListIterator();
    }

    class LinkedListIterator implements Iterator<E> {
        private Node cur = head;
        private boolean flag = true; //是否在第一圈

        @Override
        public boolean hasNext() {
            if (isEmpty()) {
                return false;
            }
            return flag;
        }

        @Override
        public E next() {
            E ret = cur.data;
            cur = cur.next;
            if (cur == head) {
                flag = false;
            }
            return ret;
        }
    }

    //双端队列的方法
    @Override
    public void addFirst(E element) {
        add(0, element);
    }

    @Override
    public void addLast(E element) {
        add(size, element);
    }

    @Override
    public E removeFirst() {
        return remove(0);
    }

    @Override
    public E removeLast() {
        return remove(size - 1);
    }

    @Override
    public E getFirst() {
        return get(0);
    }

    @Override
    public E getLast() {
        return get(size - 1);
    }

    //栈的方法
    @Override
    public void push(E element) {
        addLast(element);
    }

    @Override
    public E pop() {
        return removeLast();
    }

    @Override
    public E peek() {
        return getLast();
    }

    //队列的方法
    @Override
    public void offer(E element) {
        addLast(element);
    }

    @Override
    public E poll() {
        return removeFirst();
    }

    @Override
    public E element() {
        return getFirst();
    }
}

测试代码

package p0.测试;
import P3.链式结构.LinkedList;

import java.util.Comparator;


public class TestLinkedList {
    public static void main(String[] args) {
        //线性表
        LinkedList<Integer> list = new LinkedList<>();
        System.out.println(list);

        for (int i = 1; i <= 6; i++) {
            list.add((int) (Math.random() * 100));
        }
        System.out.println(list);
        list.sort(new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1 - o2;
            }
        });
        System.out.println(list);

        //栈
        list.push(55);
        System.out.println(list);
        System.out.println(list.pop());

    }
}

运行结果