foo.cc: In function 'int main()':
foo.cc:31:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<d.size(); i++) {
                ~^~~~~~~~~
foo.cc:32:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int j=i+1; j<d.size(); j++)
                  ~^~~~~~~~~
foo.cc:35:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<d.size(); i++) {
                ~^~~~~~~~~
foo.cc:40:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<e.size(); i++) {
                ~^~~~~~~~~
foo.cc:41:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int j=i+1; j<e.size(); j++)
                  ~^~~~~~~~~
foo.cc:44:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i=0; i<e.size(); i++) {
                ~^~~~~~~~~

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