/in/foo.cc: In function 'void back(std::__cxx11::string&)':
/in/foo.cc:16:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.length() / 2; i++)
                     ~~^~~~~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:34:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < s2.length(); i++)
                         ~~^~~~~~~~~~~~~
/in/foo.cc:36:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (i >= s1.length())
                 ~~^~~~~~~~~~~~~~
/in/foo.cc:54:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (i >= s1.length())
                 ~~^~~~~~~~~~~~~~