This problem is in Contest(s) below:
Contest 6341:基础C-第14课-课堂练习(王)
Contest 6342:基础C-第14课-课堂练习(崔)
Contest 6350:基础C-第14课-课堂练习(国)
Contest 6317:基础C-第13课-课堂练习(王)