/**

 * Definition for singly-linked list.

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int x) { val = x; }

 * }

 */

class Solution {

    public ListNode reverseKGroup(ListNode head, int k) {

        if(k == 1) return head;

        ListNode cur = head;

        ListNode curHead = head; //head of current subgroup

        ListNode nextHead = head; //head of next subgroup

        ListNode preTail = null; //tail of previous subgroup

        while(true){

            //check whether there's enough elements in subgroup

            for(int i = 1; i < k; i++){

                if(cur == null) break; //not enough element in subgroup

                cur = cur.next;

            }

            if(cur == null) break; //not enough element in subgroup

            //reverse nodes

            cur = curHead.next;

            for(int i = 1; i < k; i++){

                nextHead = cur.next;

                if(preTail==null) {

                    cur.next = head;

                    head = cur;

                }

                else {

                    cur.next = preTail.next;

                    preTail.next = cur;

                }

                cur = nextHead;

            }

            curHead.next = nextHead;

            preTail = curHead;

            curHead = nextHead;

        }

        return head;

    }

}