foo.cc: In function 'int main()':
foo.cc:10:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for(int i=1;i<a.size();i++) cnt++;
             ~^~~~~~~~~
foo.cc:11:14: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 for(int i=1;i<b.size();i++) ccnt++;
             ~^~~~~~~~~

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