/**

 * Definition for singly-linked list.

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) { val = x; }

 * }

 */

class Solution {

    public ListNode reverseList(ListNode head) {

        if (head == null) {

            return head;

        }

        ListNode pre = null;

        ListNode cur = head;

        while (cur != null) {

            ListNode next = cur.next;

            cur.next = pre;

            pre = cur;

            cur = next;

        }

        return pre;

    }

}

public static ListNode reverseLinkedList(ListNode head) {

        if (head == null || head.next == null) {

            return head;

        }

        ListNode node = reverseLinkedList(head.next);

        head.next.next = head;

        head.next = null;

        //比如1->2->3，第一次执行到这里：3->2->NULL

        return node;

    }