双向链表（Double-Linked List）

public class doubleLinkedList <Item>{
    
    private Node first;
    private Node last;
    private int itemcount;
     class Node{
        Node prev;
        Node next;
        Item item;
        
    }
    
    
    public void addFirst(Item item){
        Node oldfirst=first;
        first=new Node();
        first.item=item;
        first.next=oldfirst;
        if(oldfirst!=null){
            
            oldfirst.prev=first;
            
        }
        
        if(itemcount==0){
            
            last=first;
        }
        itemcount++;
    }
    
    
    public void addlast(Item item){
        
        Node oldlast=last;
        last=new Node();
        last.item=item;
        last.prev=oldlast;
        
        if(oldlast!=null){
            
            oldlast.next=last;
        }
        
    if(itemcount==0){
            
            last=first;
        }
    
    itemcount++;
    }
    //
    public Item delfirst(){
        if(first==null){
            throw new NullPointerException("no node linked list");
        }
        Item item=first.item;
        first=first.next;
        if(first!=null){
            
            first.prev=null;
        }
        
        if(itemcount==1){
            
            last=null;
        }
        itemcount--;
        return item;
        
        
    }
    
    
    public Item dellast(){
        
        if(last==null){
            throw new NullPointerException("no node this linked");
        }
        Item item =last.item;
        last=last.prev;
        
        if(last!=null){
            
            last.next=null;
            
        }
      if(itemcount==1){
            
            first=null;
        }
      itemcount--;
      return item;
      
    }

    //
    public void addbefore(Item targetItem,Item item){
        
        Node target=first;
        if(target==null){
            
            throw new NullPointerException("no node this linked");
        }
        
        if(target!=null && target.item!=targetItem){
            
            target=target.next;
        }
        
        if(target.prev==null){
            
            addFirst(item);
        }
        else{
            Node targetprev=target.prev;
            Node newtarget=new Node();
            newtarget.item=item;
            target.prev=newtarget;
            newtarget.next=target;
            newtarget.prev=targetprev;
            targetprev.next=newtarget;
            
            itemcount++;
        }
    }
    
    
    //
    public void addafter(Item targetItem,Item item){
    
        Node target =first;
        if(target==null){
            
            throw new NullPointerException("no node this linked!");
            
        }
        if(target!=null && target.item!=targetItem){
            target=target.next;
            
        }
        if(target.next==null){
            
            addlast(item);
        }
        else{
            
            Node oldtarget=target.next;
            Node newnode=new Node();
            newnode.item=item;
            newnode.next=oldtarget;
            oldtarget.prev=newnode;
            target.next=newnode;
            newnode.prev=target;
            
            itemcount++;
        }
        
        
    }
}