翻转链表

//  反转单链表
     public static   Node ReverseList(Node head) {
         Node next = null ; // 保留节点后驱
         Node pre = null; // 保留节点前驱     
         while(head != null){
             next = head.next;         
             head.next = pre;         
             pre = head;     
             head = next; 
         }
         return pre;
    }
//  翻转双链表
public static DoubleNode reverseDoubleList(DoubleNode head) {
        DoubleNode pre = null;
        DoubleNode next = null;
        while (head != null) {
            next = head.next;
            head.next = pre;
            head.last = next;
            pre = head;
            head = next;
        }
        return pre;
    }