void duplicate(std::vector<int>& v)
{
  for (size_t i = 0, count = v.size(); i < count; ++i)
  {
    v.push_back(v[i]);
  }
}

void duplicate(std::vector<int>& v)
{
  size_t i = 0;
  size_t count = v.size();
  for (auto const& e : v)
  {
    if (i == count)
      break;
    v.push_back(e);
    i++;
  }
}