/in/foo.cc: In function 'void add(int)':
/in/foo.cc:10:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(; i < v[k-2].size() && j < v[k-1].size(); i++,j++){
        ~~^~~~~~~~~~~~~~~
/in/foo.cc:10:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(; i < v[k-2].size() && j < v[k-1].size(); i++,j++){
                             ~~^~~~~~~~~~~~~~~
/in/foo.cc:15:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(i >= v[k-2].size()){
     ~~^~~~~~~~~~~~~~~~
/in/foo.cc:16:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(; j < v[k-1].size(); j++)
         ~~^~~~~~~~~~~~~~~
/in/foo.cc:19:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  else if(j >= v[k-1].size()){
          ~~^~~~~~~~~~~~~~~~
/in/foo.cc:20:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(; i < v[k-2].size(); i++)
         ~~^~~~~~~~~~~~~~~