/in/foo.cc: In function 'int main()':
/in/foo.cc:23:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(i=0;i<a.size();i++)
          ~^~~~~~~~~
/in/foo.cc:26:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(j=0;j<b.size();j++)
           ~^~~~~~~~~
/in/foo.cc:33:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int k=0;k<c.size();k++)
               ~^~~~~~~~~