反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

class Solution {
    public ListNode reverseList(ListNode head) {
            ListNode prev = null;
            while(head != null){
                ListNode cur = head;
                head = head.next;
                cur.next = prev;
                prev = cur;
            }
            return prev;
    }
}