foo.c:4:11: error: size of array 'f' has non-integer type
 long long f[MAX];
           ^
foo.c:4:1: warning: variably modified 'f' at file scope
 long long f[MAX];
 ^~~~
foo.c: In function 'main':
foo.c:13:11: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long long int' [-Wformat=]
  printf("%d\n",f[n]);
          ~^    ~~~~
          %lld
foo.c:9:5: 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)