/in/foo.cc: In function 'int main()':
/in/foo.cc:12:13: error: 'i' was not declared in this scope
         for(i=n-1;i>=0;i--)
             ^
/in/foo.cc:5:23: warning: unused variable 'h' [-Wunused-variable]
     int head1,back2,n,h;
                       ^
/in/foo.cc:6:10: warning: unused variable 'head' [-Wunused-variable]
     char head,back,a[50];
          ^~~~