foo.c: In function 'main':
foo.c:11:18: error: 'f' undeclared (first use in this function)
  printf("%lld\n",f[n]);
                  ^
foo.c:11:18: note: each undeclared identifier is reported only once for each function it appears in
foo.c:7:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&n);
     ^~~~~~~~~~~~~~

自豪的采用 HydroJudger 进行评测(github.com/hydro-dev/HydroJudger)