Design follow/follower classes

## Object-Oriented Design: Follow / Followers Relationship Design a small set of classes for a social product where users can **follow** and **unfollow** each other. ### Requirements - Each user can follow many other users. - Each user can be followed by many other users. - Support querying: - `getFollowers(user)` → all users who follow `user` - `getFollowing(user)` → all users whom `user` follows - `isFollowing(a, b)` → whether user `a` follows user `b` - `isMutual(a, b)` → whether `a` follows `b` **and** `b` follows `a` - Handle edge cases: - following yourself (decide behavior) - repeated follow/unfollow calls should be idempotent ### Constraints (assume) - You may assume all users are already created and identified by a unique `userId`. - You may choose in-memory data structures; optionally discuss how this maps to persistence.