Leetcode 206. Reverse Linked List

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 class Solution {
10     public ListNode reverseList(ListNode head) {
11         ListNode p0, p1;
12         p0 = null;
13         p1 = head;
14         while(p1 != null) {//确保p1不为空
15             ListNode p2 = p1.next;
16             p1.next = p0;
17             p0 = p1;
18             p1 = p2;
19         }
20         return p0;
21     }
22 }

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 class Solution {
10     public ListNode reverseList(ListNode head) {
11         if(head == null || head.next == null) return head;//这个判断很重要
12         ListNode p1 = head.next;
13         ListNode p0 = reverseList(p1);
14         p1.next = head;
15         head.next = null;
16         return p0;
17     }
18 }