Reverse Nodes in k-Group

/**
 * 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) {
        if(head == null){
            return null;
        }
        Stack<ListNode> stack = new Stack<ListNode>();
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        ListNode next = dummy.next;
        while(next!=null){
            for(int i = 0; i<k && next != null; i++){
                stack.push(next);
                next = next.next;
            }
            if(stack.size() !=k) return dummy.next;
            while(stack.size() !=0){
                cur.next = stack.pop();
                cur = cur.next;
            }
            cur.next = next;
        }
        return dummy.next;
    }
}