leetcode -- Rotate List

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

[解题思路]

初始的想法是类似于求倒数第k个数，结果发现k可能会比len大，到网上搜索了下：

从head节点开始跑，一直跑到尾节点，得到len，将尾节点的next指针指向head，接着跑(len -n) % len步，断开

这里需注意(len -n) % len可能为负数，需+len

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode rotateRight(ListNode head, int n) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         if(head == null || n == 0){
17             return head;
18         }
19         int len = 1;
20         ListNode p = head;
21         while(p.next != null){
22             len ++;
23             p = p.next;
24         }
25         p.next = head;
26         /*
27         int count = (len - n - 1) % len;
28         while(count < 0){
29             count += len;
30             count %= len;
31         }
32         p = head;
33         */
34         int count = (len - n) % len;
35         while(count < 0){
36             count += len;
37             count %= len;
38         }
39         int step = 0;
40         while(step < count){
41             p = p.next;
42             step ++;
43         }
44         head = p.next;
45         p.next = null;
46         return head;
47     }
48 }