public class Solution {
        public ListNode reverseKGroup(ListNode head, int k) {
            if (head==null||head.next==null||k<2) return head;

            ListNode dummy = new ListNode(0);
            dummy.next = head;

            ListNode tail = dummy, prev = dummy,temp;
            int count;
            while(true){
                count =k;
                while(count>0&&tail!=null){
                    count--;
                    tail=tail.next;
                }
                if (tail==null) break;


                head=prev.next;
                while(prev.next!=tail){
                    temp=prev.next;
                    prev.next=temp.next;

                    temp.next=tail.next;
                    tail.next=temp;

                }

                tail=head;
                prev=head;

            }
            return dummy.next;

        }
    }