> 3. And then write programs with vectors and maps. But the maps this time are absl::flat_hash_map (or another C++ alternative hash map such as Folly F14, etc)

> 3. And then write programs with vectors and maps. But the maps this time are absl::flat_hash_map (or another C++ alternative hash map such as Folly F14, etc) instead of std::map (or even std::unordered_map).