foo.cc: In function 'int main()':
foo.cc:8:6: warning: variable 'nn' set but not used [-Wunused-but-set-variable]
  int nn;
      ^~
foo.cc:10:6: warning: unused variable 'tmp' [-Wunused-variable]
  int tmp;
      ^~~
foo.cc:11:6: warning: unused variable 'k' [-Wunused-variable]
  int k=1;
      ^

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