public static ListNode reverseSecondHalfList(ListNode head) {
    if (head == null || head.next == null)      return head;
    ListNode fast = head;
    ListNode slow = head;
    while (fast.next != null && fast.next.next != null) {
        fast = fast.next.next;
        slow = slow.next;
    }
    ListNode pre = slow.next;
    ListNode cur = pre.next;
//Pay Attention!!别倒混了
    while (cur != null) {
        pre.next = cur.next;
        cur.next = slow.next;
        slow.next = cur;
        cur = pre.next;
    }
    return head;
}