Leetcode25 Reverse Nodes in k-Group

class Solution {
  public ListNode reverseKGroup(ListNode head, int k) {
      ListNode dummy = new ListNode(0, head);
      ListNode curDummy = dummy;
      ListNode cur = head;
      while (cur != null) {
          int num = 1;
          while (num < k && cur != null) {
              cur = cur.next;
              if (cur != null) {
                  ++num;
              }
          }
          ListNode nextHead = null;
          ListNode curtail = null;
         	// break 
          if (num == k) {
              nextHead = cur.next;
              cur.next = null;
              curtail = curDummy.next;
              curDummy.next = reverse(curDummy.next);
          } else {
              return dummy.next;
          }
        	// merge
          curDummy = curtail;
          curDummy.next = nextHead;
          cur = nextHead;
      }
      return dummy.next;
  }

  // Reverse LinkedList
  public ListNode reverse(ListNode head) {
      ListNode dummy = new ListNode(0, null);
      while (head != null) {
          ListNode tmp = head.next;
          head.next = dummy.next;
          dummy.next = head;
          head = tmp;
      }
      return dummy.next;
  }
}