public ListNode reverseList(ListNode cur) {
  ListNode prev = null;
  while (cur != null) {
      // step1: save cur.next node & set cur.next as prev
      ListNode after = cur.next;
      cur.next = prev;
      // step2: update prev as cur & update cur to saved node
      prev = cur;
      cur = after;
  }
  return prev;
}
public ListNode reverseList(ListNode head) {
    if (head == null || head.next == null) {
        return head;
    }
    ListNode p = reverseList(head.next);
    head.next.next = head;
    head.next = null;
    return p;
}