/in/foo.cc: In member function 'void Keys::solve()':
/in/foo.cc:29:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < keys.size(); i++)
                   ~~^~~~~~~~~~~~~
/in/foo.cc: In member function 'void Keys::print()':
/in/foo.cc:54:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < keys.size(); i++)
                   ~~^~~~~~~~~~~~~