@Override
    public void reduce(final NullObject key, final Iterator<Serializable> values, final ReduceEmitter<NullObject, Integer> emitter) {
        final Set<Serializable> set = new HashSet<>();
        values.forEachRemaining(set::add);
        emitter.emit(NullObject.instance(), set.size());

    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/computer/clustering/peerpressure/ClusterCountMapReduce.java