*Reverse Linked List II

 1 public class Solution {
 2     public ListNode reverseBetween(ListNode head, int m, int n) {
 3         if (m >= n || head == null) {
 4             return head;
 5         }
 6         
 7         ListNode dummy = new ListNode(0);
 8         dummy.next = head;
 9         head = dummy;
10         
11         for (int i = 1; i < m; i++) {
12             if (head == null) {
13                 return null;
14             }
15             head = head.next;
16         }
17         
18         ListNode premNode = head;
19         ListNode mNode = head.next;
20         ListNode nNode = mNode, postnNode = mNode.next;
21         for (int i = m; i < n; i++) {
22             if (postnNode == null) {
23                 return null;
24             }
25             ListNode temp = postnNode.next;
26             postnNode.next = nNode;
27             nNode = postnNode;
28             postnNode = temp;
29         }
30         mNode.next = postnNode;
31         premNode.next = nNode;
32         
33         return dummy.next;
34     }
35 }