foo.cc: In function 'int main()':
foo.cc:7:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s1.size(); i++) {
                  ~~^~~~~~~~~~~
foo.cc:11:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s2.size(); i++) {
                  ~~^~~~~~~~~~~

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