Reverse Nodes in k-Group

 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 public class Solution {
10     public ListNode reverseKGroup(ListNode head, int k) {
11         if (head == null || head.next == null || k <= 1) {
12             return head;
13         }
14         ListNode dummy = new ListNode(0);
15         dummy.next = head;
16         head = dummy;
17         while (head.next != null) {
18             head = reverseNextK(head, k);
19         }
20         return dummy.next;
21     }
22     
23     //reverse head-->n1-->n2-->..->nk-->nNext
24     //to head-->nk-->nk-1-->..-->n1-->nNext
25     //return n1
26     private ListNode reverseNextK(ListNode head, int k) {
27         ListNode next = head;
28         //check  there is enought nodes to reverse
29         for (int i = 0; i < k; i++) {
30             if (next.next == null) {
31                 return next;
32             }
33             next = next.next;
34         }
35         
36         //reverse
37         ListNode node = head.next;
38         ListNode preNode = head, curt = head.next;
39         for (int i = 0; i < k; i++) {
40             ListNode temp = curt.next;
41             curt.next = preNode;
42             preNode = curt;
43             curt = temp;
44         }
45         head.next = preNode;
46         node.next = curt;
47         return node;
48     }
49 }