LeetCode Reverse Linked List II

 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 reverseBetween(ListNode head, int m, int n) {
14             if (head==null) {
15                 return null;
16             }
17             if(n==m){
18                 return head;
19             }
20             ListNode temp=head,doRe = null,second=null,first=null,last=null;
21             int i=1;
22             
23             while (temp!=null) {
24                 if (i==m-1) {
25                     first=temp;
26                 }
27                 if (i==m) {
28                     doRe=temp;
29                 }
30                 if (i==n) {
31                     last=temp;
32                     second=temp.next;
33                     if(first!=null) first.next=last;
34                     last.next=null;
35                     
36                     break;
37                 }
38                 ++i;
39                 temp=temp.next;
40             }
41             reverse(doRe);
42             doRe.next=second;
43             if (m>1) {
44                 return head;
45             }else {
46                 return last;
47             }
48             
49     }
50     private void reverse(ListNode head) {
51         ListNode pre = null,curr,next;
52 
53         curr=head;
54         if (head==null) {
55             return;
56         }
57         while (curr!=null) {
58             next=curr.next;
59             curr.next=pre;
60             pre=curr;
61             curr=next;
62         }
63     }
64 }