bst_count_map.cpp

static long long lazy_map(std::unordered_map<int, long long>& memo, int n)
{
   auto& computed = memo[n];
   if (computed) return computed;
   auto recur = [&](int k) { return lazy_map(memo, k); };
   return computed = bst_count(recur, n);
}

long long bst_count_map(int n)
{
   std::unordered_map<int, long long> memo;
   return lazy_map(memo, n);
}