foo.cc: In function 'int main()':
foo.cc:15:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.length();i++){
                 ~^~~~~~~~~~~
foo.cc:30:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.length();i++){
                 ~^~~~~~~~~~~

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