This problem is in Contest(s) below:
Contest 5656:寒假基础C-第7课-课堂练习(王)
Contest 5602:寒假基础C-第2课-课堂练习(崔)
Contest 5638:寒假基础C-第2课-课堂练习(王)
Contest 5640:寒假基础C-第2课-课堂练习(崔)
Contest 5641:寒假基础C-第2课-课堂练习(国)