LeetCode - Reverse Nodes in k-Group

package list;

public class ReverseNodesInKGroup {

    public ListNode reverseKGroup(ListNode head, int k) {
        if (head == null || k < 2) return head;
        ListNode first = new ListNode(0);
        first.next = head;
        
        ListNode prevp = first;
        ListNode p = head;
        ListNode q = p;

        while (q != null) {
            int i = 0;
            for (; q != null && i < k - 1; ++i) {
                q = q.next;
            }
            
            if (i == k - 1 && q != null) {
                ListNode qNext = q.next;
                q.next = null;
                ListNode newHead = reverseList(p);
                prevp.next = newHead;
                prevp = p;
                p = qNext;
                q = qNext;
                prevp.next = qNext;
            } 
        }
                
        return first.next;
    }
    
    // This function will only be invoked when the list contains more than one node.
    private ListNode reverseList(ListNode head) {
        ListNode prevp = null;
        ListNode p = head;
        ListNode q = head.next;
        while (p != null && q != null) {
            ListNode tmp = q.next;
            q.next = p;
            p.next = prevp;
            prevp = p;
            p = q;
            q = tmp;
        }
        
        return p;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ListNode a1 = new ListNode(1);
        ListNode a2 = new ListNode(2);
        ListNode a3 = new ListNode(3);
        ListNode a4 = new ListNode(4);
        ListNode a5 = new ListNode(5);
        a1.next = a2;
        a2.next = a3;
        a3.next = a4;
        a4.next = a5;
        a5.next = null;
        
        ReverseNodesInKGroup r = new ReverseNodesInKGroup();
        ListNode x = r.reverseKGroup(a1, 3);
        while (x != null) {
            System.out.println(x.val);
            x = x.next;
        }
    }

}