This problem is in Contest(s) below:
Contest 2987:入门-第2课-课堂练习(崔)
Contest 2994:入门-第2课-课堂练习(王老师)
Contest 2967:入门-第1课-课堂练习(秋季)
Contest 2989:9.14晚上
Contest 2997:入门-第1课-课堂练习(三小)