Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

You may not alter the values in the nodes, only nodes itself may be changed.

Only constant memory is allowed.

For example,
Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

/**
 * 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) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        
        ListNode curt = dummy;
        while(curt != null) {
            curt = reverseK(curt, k);
        }
        
        return dummy.next;
    }
    
    // head -> n1 -> n2 ... nk -> nk+1
    // =>
    // head -> nk -> nk-1 .. n1 -> nk+1
    // return n1
    public ListNode reverseK(ListNode head, int k) {
        ListNode nk = head;
        for (int i = 0; i < k; i++) {
            if(nk == null) {
                return null;
            }
            nk = nk.next;
        }
        if (nk == null) {
            return null;
        }
        
        //reverse
        //head -> n1 -> n2 ... nk - 1 -> nk -> nk+1
        ListNode n1 = head.next;
        ListNode nkplus = nk.next;
        
        ListNode prev = null;
        ListNode curt = n1;
        while (curt != nkplus) {
            ListNode temp = curt.next;
            curt.next = prev;
            prev = curt;
            curt = temp;
        }
        // null<-n1 <- n2 ... nk - 1 <- nk      nk+1
        //connect 
        head.next = nk;
        n1.next = nkplus;
        
        return n1;
    }
}