Versioned Social Network with Snapshots

Design a social network whose follow graph supports point-in-time snapshots: follow / unfollow apply to the current version, createSnapshot freezes a version, and isFollowing queries any historical snapshot. A snapshot/MVCC problem. Requirements Design a social network that supports point-in-time snapshots of its follow graph. follow(followerId, followeeId) — follower follows followee (directed, one-way). unfollow(followerId, followeeId) — remove a follow edge. createSnapshot() -> int — record the current state of all follow relationships and return a snapshot/version id. isFollowing(followerId, followeeId, snapId) -> bool — whether the edge existed in the given snapshot. A new version is created only when createSnapshot() is called. follow / unfollow apply to the current (latest) version. Historical snapshots must stay queryable even after later unfollows. Examples sn.follow(1, 2) // writes to version 0 sn.createSnapshot() // returns 0; current version becomes 1 sn.unfollow(1, 2) // writes to version 1 sn.isFollowing(1, 2, 0) // true sn.isFollowing(1, 2, 1) // false Notes This is a snapshot / MVCC problem. Instead of copying the whole graph on every snapshot, store per edge a version history: for each (follower, followee) keep an ordered list of (version, isFollowing) changes, and answer isFollowing(..., snapId) by binary searching for the latest change at or before snapId. The write version only advances when createSnapshot() is called, so multiple edits to the same edge between snapshots collapse into one version — make sure repeated edits to an edge in a single version overwrite rather than append duplicate entries. Clarify that snapshot ids are dense integers starting at 0 (as in the example) so versions can be indexed directly. Preparation Implement the per-edge version-history map with binary search over versions; this generalizes the Snapshot Array pattern (LC 1146) from a single value to a set of edges. Test the example trace exactly, plus an edge that is followed, snapshotted, unfollowed, and re-followed across several versions.