/in/foo.cc: In function 'std::__cxx11::string convert(std::__cxx11::string, int)':
/in/foo.cc:9:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (s.size() < numRows || numRows == 1)
         ~~~~~~~~~^~~~~~~~~
/in/foo.cc:14:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.size(); i++)
                     ~~^~~~~~~~~~