实现一个基于双向链表的双端队列

package day1_30_5_2;

public class DoubleLinkedList {

	private Link first;
	private Link last;

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

	//判断是不是为空
	public boolean isEmpty(){
		return first == null;
	}

	//在双向链表的前端插入
	public void insertFirst(long dd){
		Link newLink = new Link(dd);
		if(isEmpty()){//如果双向链表为空，就让last指针直接指向新创建的节点
			last = newLink;
		}else
			first.previous = newLink;
		    newLink.next = first;
		    first = newLink;

	}

	//在双向链表的后端插入
	public void insertLast(long dd){
		Link newLink = new Link(dd);
		if(isEmpty()){
			first = newLink;
		}else{
			last.next = newLink;
			newLink.previous = last;

		}
		last = newLink;

	}

	//删除第一个节点的实现
	public Link deleteFirst(){

		Link temp = first;
		if(first.next == null){//如果只有一个元素
			last = null;
		}
		else
			first.next.previous = null;
		    first = first.next;
		    return temp;

	}

	//删除最后一个节点的实现
	public Link deleteLast(){
		Link temp = last;
		if (first.next == null) // if only one item
            first = null; // first --> null
        else
            last.previous.next = null; // old previous --> null
        last = last.previous; // old previous <-- last
        return temp;
	}

	//在某个元素的后面添加一个元素,insert dd after key
	public boolean insertAfter(long key,long dd){
		Link current = first;

		while(current.dData != key){
			current = current.next;
			if(current == null){
				return false;
			}
		}
		//创建一个新的节点
		Link newLink = new Link(dd);
		//current  在last
		if(current == last){
			newLink.next = null;
			last = newLink;
		}else{//不是最后一个节点
			newLink.next = current.next;
			current.next.previous = newLink;

		}

		newLink.previous = current;
		current.next = newLink;
		return true;
	}

	public Link deleteKey(long key){

		 Link current = first; // start at beginning
	        while (current.dData != key) // until match is found,
	        {
	            current = current.next; // move to next link
	            if (current == null)
	                return null; // didn‘t find it
	        }
	        if (current == first) // found it; first item?
	            first = current.next; // first --> old next
	        else
	            // not first
	            // old previous --> old next
	            current.previous.next = current.next;  

	        if (current == last) // last item?
	            last = current.previous; // old previous <-- last
	        else
	            // not last
	            // old previous <-- old next
	            current.next.previous = current.previous;
	        return current; // return value  

	}

	public void displayForward() {
        System.out.print("List (first-->last): ");
        Link current = first; // start at beginning
        while (current != null) // until end of list,
        {
            current.displayLink(); // display data
            current = current.next; // move to next link
        }
        System.out.println("");
    }  

    // -------------------------------------------------------------
    public void displayBackward() {
        System.out.print("List (last-->first): ");
        Link current = last; // start at end
        while (current != null) // until start of list,
        {
            current.displayLink(); // display data
            current = current.previous; // move to previous link
        }
        System.out.println("");
    }  

}