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) {
return null;
}
if (k <= 1) {
return head;
}
ListNode dummy = new ListNode(-1);
dummy.next = head;
ListNode start = dummy;
Boolean isLast = false;
while (true) {
ListNode cur = start;
for (int i = 0; i < k; i++) {
cur = cur.next;
if (cur == null) {
isLast = true;
break;
}
}
if (isLast) {
break;
}
start = myReverse(start, k);
}
return dummy.next;
}

private ListNode myReverse(ListNode start, int k) {
ListNode ptr = start.next;
for (int i = 1; i < k; i++) {
ListNode temp = ptr.next;
ptr.next = temp.next;
temp.next = start.next;
start.next = temp;
}
return ptr;
}
}