Reverse Nodes in k-Group

Source

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

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

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    /**
     * @param head a ListNode
     * @param k an integer
     * @return a ListNode
     */
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode cur = head;
        int pos=1;

        while(cur!=null){
            if(pos==k){
                pre = reverse(pre, cur);
                cur=pre.next;
                pos=1;

            }
            else{
                pos++;
                cur=cur.next;
            }
        }
        return dummy.next;
    }

    public ListNode reverse(ListNode pre, ListNode end){
        ListNode cur = pre.next;
        while(pre.next!=end){
            ListNode tmp = cur.next.next;  
            cur.next.next = pre.next;  
            pre.next = cur.next;  
            cur.next = tmp;              
        }
        return cur; 
    }
}