/in/foo.cc: In function 'void mul(std::__cxx11::string&, std::__cxx11::string)':
/in/foo.cc:5:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s1.length() / 2; i++) swap(s1[i], s1[s1.length() -1 - i]);
                 ~~^~~~~~~~~~~~~~~~~
/in/foo.cc:6:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s2.length() / 2; i++) swap(s2[i], s2[s2.length() -1 - i]);
                 ~~^~~~~~~~~~~~~~~~~
/in/foo.cc:8:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s1.length(); i++)
                 ~~^~~~~~~~~~~~~
/in/foo.cc:9:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j = 0; j < s2.length(); j++)
                  ~~^~~~~~~~~~~~~
/in/foo.cc:12:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s1.length() + s2.length() - 1; i++)
                 ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:14:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s1.length() + s2.length(); i++) s3 += (ans[s1.length() + s2.length() - 1 - i]) + '0';
                 ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~