foo.cc: In function 'int main()':
foo.cc:6:9: warning: 'i' is used uninitialized in this function [-Wuninitialized]
  int a[i];
         ^
foo.cc:21:8: warning: 'xb' may be used uninitialized in this function [-Wmaybe-uninitialized]
  cout<<xb<<endl;
        ^~

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