foo.cc: In function 'int main()':
foo.cc:12:2: error: 'cin' was not declared in this scope
  cin>>n;
  ^~~
foo.cc:12:2: note: suggested alternative: 'main'
  cin>>n;
  ^~~
  main
foo.cc:37:2: error: 'cout' was not declared in this scope
  cout<<res<<endl;
  ^~~~
foo.cc:37:13: error: 'endl' was not declared in this scope
  cout<<res<<endl;
             ^~~~
foo.cc:37:13: note: suggested alternative: 'enum'
  cout<<res<<endl;
             ^~~~
             enum

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