This problem is in Contest(s) below:
Contest 5973:基础C-第5课-课堂练习(国)
Contest 5976:基础C-第5课-课堂练习(王)
Contest 5977:基础C-第5课-课堂练习(崔)
Contest 6053:基础C-第7课-课堂练习(崔)
Contest 6054:基础C-第7课-课堂练习(王)
Contest 6055:基础C-第7课-课堂练习(国)
Contest 5993:基础班4-7