Design a Social Network Class with Follow Relationships and Versioned Snapshots

Design a Social Network class that tracks follower relationships between users. Implement the following operations: follow(userA, userB), unfollow(userA, userB), snapshot() which captures the current state and returns a snapshot ID, isFollow(userA, userB, snapshotId) to query whether a follow relationship existed at a given snapshot, getFollowees(userId, snapshotId) to retrieve users that a given user followed at a snapshot, and getUsersFollowed(userId, snapshotId) to retrieve users who followed a given user at a snapshot.