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();
     }
 }