[Boost-users] [Unordered] Penultimate Iterator

Dear all, What is the quickest way to get hold of the penultimate iterator in an unordered_map? I'm aware of this one -- which is O(n), where n == container_.size(): container_type::const_iterator i = container_.begin(); advance(i, container_.size() - 1); Is there no O(1) solutions? TIA, --Hossein