/in/foo.cc: In function 'int main()':
/in/foo.cc:7:13: error: 'cin' was not declared in this scope
     int n;  cin >> n;
             ^~~
/in/foo.cc:26:5: error: 'cout' was not declared in this scope
     cout << ans.size() << "\n";
     ^~~~
/in/foo.cc:27:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0; i<ans.size(); ++i)
                  ~^~~~~~~~~~~