/in/foo.cc: In function 'int main()':
/in/foo.cc:29:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<str.length();i++){
              ~^~~~~~~~~~~~~
/in/foo.cc:37:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<V[i].size();j++)
               ~^~~~~~~~~~~~