This problem is in Contest(s) below:
Contest 2783:入门-第14课-课堂练习(雷老师)
Contest 3341:入门-第14课-课堂练习(童)
Contest 3345:入门-第14课-课堂练习(王老师)
Contest 3348:入门-第14课-课堂练习(崔)