92. Reverse Linked List II

题目链接：https://leetcode.com/problems/reverse-linked-list-ii/
解题思路：
 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 reverseBetween(ListNode head, int m, int n) {
11         
12         
13         ListNode newhead = new ListNode(-1);
14         newhead.next  = head;
15         
16         if(head==null || head.next ==null)
17             return head;
18         
19         ListNode startpoint = newhead;
20         ListNode node1 = null;
21         ListNode node2 = null;
22         
23         for(int i=0;i<n;i++)
24         {
25             if(i<m-1)
26             {
27                 startpoint = startpoint.next;
28             }
29             else if(i==m-1)
30             {
31                 node1 = startpoint.next;
32                 node2 = node1.next;
33             }
34             else
35             {
36                 node1.next = node2.next;
37                 node2.next = startpoint.next;
38                 startpoint.next = node2;
39                 node2 = node1.next;
40             }
41         }
42         return newhead.next;
43         
44     }
45 }