[数据结构与算法]双向链表

虽然从表内第一个节点到最后一个节点的遍历操作是非常简单的，但是反向遍历链表却不是一件容易的事情。如果为Node类添加一个字段来存储指向前一个节点的连接，那么久会使得这个反向操作过程变得容易许多。当向链表插入节点的时候，为了吧数据复制给新的字段会需要执行更多的操作，但是当腰吧节点从表移除的时候就能看到他的改进效果了。
首先需要修改Node类来为累增加一个额外的链接。为了区别两个连接，这个把指向下一个节点的链接称为FLink,而吧指向前一个节点的连接称为BLink.在实例化一个Node的时候，会把这些字段都设置为nothing.

   public class LinkedList
    {
       protected Node header;
       public LinkedList() 
       {
           header = new Node("header");
       }
       private Node Find(Object item)
       {
           Node current = new Node();
           current = header;
           while (current.Element != item) 
           {
               current = current.FLink;
           }
           return current;
       }
       public void Insert(Object newItem,Object after) 
       {
           Node current = new Node();
           Node newNode = new Node(newItem);

           current = Find(after);
           newNode.FLink = current.FLink;
           newNode.Blink = current;
           current.FLink = newNode; 

       }
       public void Remove(Object n) 
       {
           Node p = Find(n);

           if (!(p.FLink == null)) 
           {
               p.Blink.FLink = p.FLink;
               p.FLink.Blink = p.Blink;

               p.FLink = null;
               p.Blink = null;

           }
       }
       private Node FindLast() 
       {
           Node current = new Node();
           current = header;
           while (!(current.FLink == null)) 
           {
               current = current.FLink;
           }
           return current;
       }
       public void PrintList() 
       {
           Node current = new Node();
           current = header;

           while (!(current.FLink == null))
           {
               Console.WriteLine(current.Element);
               current = current.FLink;
           }
       }
       public void PrintReverse() 
       {
           Node current = new Node();
           current = FindLast();

           while (!(current.Blink == null)) 
           {
               Console.WriteLine(current.Element);
               current = current.Blink;
           }
       }

    }