Maps inversion

gistfile1.txt

   public <A, B, C> Map<B, List<Pair<A, C>>> invertMap(Map<A, Map<B, C>> input) {
        return input.entrySet().stream()
                .flatMap(kv -> kv.getValue().entrySet().stream().map(vs -> new Pair<>(kv.getKey(), vs)))
                .collect(Collectors.groupingBy(
                        it -> it.getValue().getKey(),
                        Collectors.mapping(
                                it -> new Pair<>(it.getKey(), it.getValue().getValue()),
                                Collectors.toList()
                        ))
                );
  }