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("");      }  					}