foo.cc: In function 'int main()':
foo.cc:6:10: error: conflicting declaration 'int a [11]'
  int a[11];
          ^
foo.cc:5:6: note: previous declaration as 'int a'
  int a,b,c=0,d;
      ^
foo.cc:9:11: error: invalid types 'int[int]' for array subscript
   cin>>a[i];
           ^
foo.cc:15:10: error: invalid types 'int[int]' for array subscript
 if(b<=a[i]+30)
          ^
foo.cc:5:14: warning: unused variable 'd' [-Wunused-variable]
  int a,b,c=0,d;
              ^

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