/in/foo.cc: In function 'int main()':
/in/foo.cc:54:2: error: 'sort' was not declared in this scope
  sort(a+1, a+1+n, cmp);
  ^~~~
/in/foo.cc:54:2: note: suggested alternative: 'qsort'
  sort(a+1, a+1+n, cmp);
  ^~~~
  qsort