双向链表--Java实现

/*双向链表特点：
 *1.每个节点含有两个引用，previos和next,支持向前或向后的遍历(除头节点)
 *2.缺点插入或删除的时候涉及到引用修改的比较多
 *注意：下面的双向链表其实也实现了双端链表
 *注意：在Java中多个引用可以指向同一个对象，也可以随时改变引用的指向
 *      关于修改引用细心一点就可以 引用A = 引用B 表示A引用指向B引用指向的对象
 *应用:利用双向链表可以实现双端队列
 * */

public class MyDoubleLink {
    private Link first;
    private Link last;
    
    public boolean isEmpty(){
        return first == null;
    }
    
    public void insertFirst(int key){
        Link newLink = new Link(key);
        if(first == null){
            last = newLink;
        }
        else{
            first.previous = newLink;
        }
        newLink.next = first;//链未断可以指向同一个
        first = newLink;
    }
    
    public void insertLast(int key){
        Link newLink = new Link(key);
        if(first == null){
            first = newLink;
        }
        else{
            last.next = newLink;
            newLink.previous = last;
        }
        last = newLink;
    }
    
    //插入指定值的后边---其实是一种尾巴插入--此时链表非空才可以操作
    public boolean insertAfter(int key,int value){
        Link newLink = new Link(value);
        Link current = first;
        while(current.id != key){
            current = current.next;
            if(current == null){
                return false;
            }
        }
        if(current == last){//find it at last item
            newLink.next = null;
            last = newLink;
        }
        else{
            newLink.next = current.next;
            current.next.previous = newLink;
        }
        newLink.previous = current;
        current.next = newLink;
        return true;
    }
    
    public Link deleteFirst(){
        Link temp = first;
        if(first.next == null){
            last = null;
        }
        else{
            first.next.previous = null;
        }
        first = first.next;
        return temp;
    }
    
    public Link deleteLast(){
        Link temp = last;
        if(first.next == null){
            first = null;
        }
        else{
            last.previous.next = null;
        }
        last = last.previous;
        return temp;
    }
    
    //按照值进行删除--可能存在找不到的时候
    public Link delete(int key){
        Link current = first;
        while(current.id != key ){
            current = current.next;
            if(current == null){
                return null;
            }
        }
        if(current == first){//find it at first item并非只有一个节点
            first = current.next;
        }
        else{  //find it not first item
            current.previous.next = current.next;
        }
        
        if(current == last){  //find it at last item
            last = current.previous;
        }
        else{ //find it not last
            current.next.previous = current.previous;
        }
        return current;
    }
    
    public void diaplayFirstToLast(){
        System.out.println("first to last");
        Link current = first;
        while(current != null){
            System.out.print(current.id + " ");
            current = current.next;
        }
        System.out.println();
    }
    
    public void displayLastToFirst(){
        System.out.println("last to first");
        Link current = last;
        while(current != null){
            System.out.print(current.id + " ");
            current = current.previous;
        }
        System.out.println();
    }
}