foo.cc: In function 'int main()':
foo.cc:5:8: warning: unused variable 'j' [-Wunused-variable]
  int n,j;scanf("%d",&n);
        ^
foo.cc:5:15: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  int n,j;scanf("%d",&n);
          ~~~~~^~~~~~~~~

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