This problem is in Contest(s) below:
Contest 5845:基础B-第2课-课堂练习(崔)
Contest 5843:基础B-第2课-课堂练习(童)
Contest 5811:春季基础B-第1课-课堂练习(童)
Contest 5813:春季基础B-第1课-课堂练习(崔)