This problem is in Contest(s) below:
Contest 1891:字符串练习
Contest 1578:2022国庆集训收官之战
Contest 1803:字符串加强练习
Contest 1970:语法提高-字符串练习-(2)字符串综合练习
Contest 1942:字符串基础练习（1）
Contest 1651:字符练习