Design a system for recording information about songs listened by user and providing top listen chart for the last week.

You are given two APIs that limit interaction with this system (provided by interviewer):

recordListen(UserID userID, SongID songID)
List<SongID> getTopListens(UserID userID)

You can only get top listens for last week.

There will be 10M total users initially and 75% are active daily. The total number of songs is 50M.