foo.cc: In member function 'void Arry::charu()':
foo.cc:32:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for(int i=0;i<a.size();i++) cout<<a[i]<<" ";
                ~^~~~~~~~~

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