/in/foo.cc: In function 'int change(const std::string&, const std::string&)':
/in/foo.cc:22:14: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   22 |         if(i > str2.size())    //如果循环节长度大于要连接的单词,说明在此长度及以后长度无解
      |            ~~^~~~~~~~~~~~~