Reverse Nodes in k-Group (LeetCode 25)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0, head);  // create a dummy node to return its next (head) at the end
        ListNode prev = dummy;
        while (true) {
            ListNode tail = prev;
            int count = 0;
            while (tail != null && count < k) {
                tail = tail.next;
                count ++;
            }
            if(tail == null) {  // break if the current node (tail) becomes null
                break;
            }
            ListNode start = prev.next;
            ListNode nextStart = tail.next;
            prev.next = reverseList(start, tail);
            prev = start;
        }
        return dummy.next;
    }

    private ListNode reverseList(ListNode start, ListNode end) {
        ListNode prev = end.next;
        while (start != end) {
            ListNode next = start.next;
            start.next = prev;
            prev = start;
            start = next;
        }
        start.next = prev;
        
        return end;
    }
}