/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public void reverse(ListNode head)
    {
        ListNode first=head;
        ListNode second=first.next;
        first.next=null;
        while(second!=null)
        {
            ListNode p=second.next;
            second.next=first;
            first=second;
            second=p;
        }
    }
    public ListNode reverseKGroup(ListNode head, int k) {
        if(head==null||head.next==null||k<=1)
        {
            return head;
        }
        ListNode pHead=new ListNode(0);
        ListNode first=null;
        ListNode end=null;
        ListNode p=null;
        ListNode last=pHead;
        for(first=head;first!=null;first=p)
        {
            int i=1;
            for(end=first;end.next!=null&&i<k;i++,end=end.next);
            if(i<k)
            {
                last.next=first;
                break;
            }
            p=end.next;
            end.next=null;
            reverse(first);
            last.next=end;
            last=first;
        }
        return pHead.next;
    }
}