/in/foo.cc: In function 'std::__cxx11::string cheng(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:12:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < x.size(); i ++)f[x.size() - i] = x[i] - '0';
                 ~~^~~~~~~~~~
/in/foo.cc:13:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < y.size(); i ++)d[y.size() - i] = y[i] - '0';
                 ~~^~~~~~~~~~
/in/foo.cc:16:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 1; i <= y.size(); i ++){
                 ~~^~~~~~~~~~~