反转链表

// 反转prev的后面，并且返回尾指针
ListNode reverse(ListNode prev) {
    ListNode curr = prev.next;
    while (curr.next != null) {
        ListNode next = curr.next;
        curr.next = next.next;
        next.next = prev.next;
        prev.next = next;
    }
    return curr;
}