This problem is in Contest(s) below:
Contest 1282:昌乐二中基础组20210810
Contest 1496:基础语言提高班-4
Contest 1702:2023基础提高班6
Contest 1942:字符串基础练习(1)
Contest 1969:语法提高-字符串练习-(1)字符串基础
Contest 1986:2025冬令营语言提高-6(结构体)
Contest 1781:查漏补缺基础5
Contest 1730:2023基础提高班第三天
Contest 1754:2023语言提高班(练习)
Contest 1651:字符练习