foo.cc: In function 'int main()':
foo.cc:32:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&n);
  ~~~~~^~~~~~~~~
foo.cc:33:41: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  for(int i=1;i<=n;i++ )scanf("%d",&a[i]),pre[i]=pre[i-1]^a[i];
                        ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

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

{"receive":"2020-07-25T14:19:10.159Z","handle":"2020-07-25T14:19:10.159Z","cache_start":"2020-07-25T14:19:10.167Z","read_cases":"2020-07-25T14:19:10.281Z","judge":"2020-07-25T14:19:10.288Z","done":"2020-07-25T14:19:12.273Z"}