foo.cc: In function 'int main()':
foo.cc:10:2: error: 'sort' was not declared in this scope
  sort(a+1,a+n+1);
  ^~~~
foo.cc:10:2: note: suggested alternative: 'qsort'
  sort(a+1,a+n+1);
  ^~~~
  qsort

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