https://www.programmingsimplified.com/c/program/c-program-change-case
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/strlwr-s-strlwr-s-l-mbslwr-s-mbslwr-s-l-wcslwr-s-wcslwr-s-l?view=vs-2019
原文：https://www.cnblogs.com/liujx2019/p/11868739.html