Reverse a singly linked list.
Example：
    Input：1->2->3->4->5->NULL
    Output：5->4->3->2->1->NULL

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
                ListNode pre=null;
                ListNode next=null;

                while(head!=null){
                    next=head.next;
                    head.next=pre;
                    pre=head;
                    head=next;
                }
        //此时注意：1、关于指针的是head.next指向pre
                2、关于赋值的是把后面的值赋给前面，比如pre=head,是把head的值赋给pre                        
                return pre;
    }
}

/**
 * Definition for singly-linked list.
 * public class DoubleNode {
 *     int val;
 *     DoubleNode last;
 *     DoubleNode next;
 *     DoubleNode(int x) { val = x; }
 * }
 */
class Solution {
    public DoubleNode reverseList(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;
    }
}