recursion_v2.cpp

std::vector<std::string> recursion(std::vector<char> chars, int min_rec, int max_rec)
{
  std::vector<std::string> results;

  if (min_rec <= 1)
  {
    for (char c : chars)
    {
      results.push_back(std::string(1, c));
    }
  }

  if (max_rec == 1)
  {
    return results;
  }

  auto rec = recursion(chars, min_rec - 1, max_rec - 1);

  for (auto c : chars)
  {
    for (auto s2 : rec)
    {
      results.push_back(c + s2);
    }
  }

  return results;
}