/**

* Definition for singly-linked list.

* public class ListNode {

* int val;

* ListNode next;

* ListNode(int x) {

* val = x;

* next = null;

* }

* }

*/

public class Solution {

class Bundle {

public ListNode beg;

public ListNode end;

Bundle(ListNode beg, ListNode end){

this.beg = beg;

this.end = end;

}

}

private Bundle reverseBet(Bundle b){

if(b.beg==b.end) return b;

ListNode pPre = null;

ListNode p = b.beg;

ListNode tmp;

while(p!=b.end){

tmp = p.next;

p.next = pPre;

pPre = p;

p = tmp;

}

p.next = pPre;

return new Bundle(b.end,b.beg);

}

public ListNode reverseKGroup(ListNode head, int k) {

// Start typing your Java solution below

// DO NOT write main() function

ListNode ans = head;

ListNode p = head;

ListNode pPre = new ListNode(0);

pPre.next = head;

Bundle lastBundle = new Bundle(pPre,pPre);

int no = 0;

while(p!=null){

no++;

if(no==k){

no=0;

ListNode pnext = p.next;

Bundle kb = reverseBet(new Bundle(lastBundle.end.next,p));

lastBundle.end.next = kb.beg;

kb.end.next = pnext;

lastBundle = kb;

p = kb.end;

}

p = p.next;

}

return pPre.next;

}

}