foo.cc: In function 'int main()':
foo.cc:19:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<a.size(); i++) {
                ~^~~~~~~~~
foo.cc:20:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int j=i+1; j<a.size(); j++)
                  ~^~~~~~~~~
foo.cc:23:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<a.size(); i++) {
                ~^~~~~~~~~
foo.cc:28:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<b.size(); i++) {
                ~^~~~~~~~~
foo.cc:29:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int j=i+1; j<b.size(); j++)
                  ~^~~~~~~~~
foo.cc:32:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<b.size(); i++) {
                ~^~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。