/**
 *
 * 双端链表
 */
class Link {
 
    public long dData;
    public Link next;

    public Link(long d) {
        dData = d;
    }

    public void displayLink() {
        System.out.print(dData + " ");
    }
}

class FirstLastList {

    private Link first;
    private Link last;

    public FirstLastList() {
        first = null;
        last = null;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public void insertFirst(long dd) {
        Link newlink = new Link(dd);

        if (isEmpty()) {
            last = newlink;
        }
        newlink.next = first;
        first = newlink;
    }
    
    //这里看不懂，不知道为什么插入last，然后他插入的值会在first里面,displayList()方法可以打印出first的值,first和last好像都不是同一个Link
    public void insertlast(long dd) {
        Link newlink = new Link(dd);
        if (isEmpty()) {
            first = newlink;
        } else {
            last.next = newlink;
        }
        last = newlink;
    }

    public long deleteFirst() {
        long temp = first.dData;
        if (first.next == null) {
            last = null;
        }
        first = first.next;
        return temp;
    }

    public void displayList() {
        System.out.print("List(first-->last):");
        Link current = first;
        while(current != null){
            current.displayLink();
            current = current.next;
        }
        System.out.println("");
    }
}

public class Test4 {
    public static void main(String[] args){
        FirstLastList thelist = new FirstLastList();
        
        thelist.insertFirst(22);
        thelist.insertFirst(44);
        thelist.insertFirst(66);
        
        thelist.insertlast(11);
        thelist.insertlast(33);
        thelist.insertlast(55);
    
        thelist.displayList();
        
        thelist.deleteFirst();
        thelist.deleteFirst();
        thelist.displayList();

        
    }
}