bst_count_open_recursive.cpp

template<typename Recur>
long long bst_count(Recur recur, int n)
{
   if (n <= 1) return 1;

   long long sub_counts = 0;
   for (int i = 0; i < n; ++i)
      sub_counts += recur(i) * recur(n-1-i);
   return sub_counts;
}