From Berlin Brandenburg Airport (BER) to the apartment: 

(40 minutes) RE8 → S+U Alexanderplatz, change to M4 → Hufelandstraße, walk 2 minutes to the apartment

(46 minutes) FEX → S Ostkreuz, change to S42 → S Greifswalder Straße, change to M4 → Hufelandstraße, walk 2 minutes to the apartment

(59 minutes) S9 → S+U Alexanderplatz, change to M4 → Hufelandstraße, walk 2 minutes to the apartment

From Berlin Central Station to the apartment:

(20 minutes) S3 → S+U Alexanderplatz, change to M4 → Hufelandstraße, walk 2 minutes to the apartment

From Berlin Central Bus Station (ZOB) to the apartment:

(43 minutes) walk 12 minutes to S Messe Nord/ZOB, S41 → S Greifswalder Straße, change to M4 → Hufelandstraße, walk 2 minutes to the apartment

(50 minutes) walk 12 minutes to S Messe Nord/ZOB, S42 → S Westkreuz, change to S7 → S+U Alexanderplatz, change to M4 → Hufelandstraße, walk 2 minutes to the apartment