Reverse Nodes in k-Group

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
public ListNode reverseKGroup(ListNode head, int k) {
if (head == null || k <= 1) {
return head;
}
ListNode dummy = new ListNode(0);
ListNode prev = dummy;
ListNode node = head;
dummy.next = head;
int count = 0;
while (node != null) {
count++;
ListNode next = node.next;
if (count == k) {
prev = helper(prev, next);
count = 0;
}
node = next;
}
return dummy.next;
}

private ListNode helper(ListNode prev, ListNode next){
ListNode last = prev.next;
ListNode cur = last.next;
while (cur != next) {
last.next = cur.next;
cur.next = prev.next;
prev.next = cur;
cur = last.next;
}
return last;
}
}