/in/foo.cc: In function 'int main()':
/in/foo.cc:24:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0; i < u1.size(); i++) cout << u1[i] << " ";
                    ~~^~~~~~~~~~~
/in/foo.cc:29:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0; i < u2.size(); i++) cout << u2[i] << " ";
                    ~~^~~~~~~~~~~