Social Network with Immutable Snapshots + Followers Index + Recommendations

Question

## Social Network with Point-in-Time Snapshots

### Context
You are building an in-memory social network. Users can follow each other (directed edges). The system must support creating immutable, point-in-time snapshots of the follow graph, and querying those snapshots for relationship checks, follower/following lists, and follow recommendations.

You will implement this incrementally across three parts.

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## Part 1: Users, Follows, and Immutable Snapshots

### Requirements
Implement a `SocialNetwork` class that manages:
- user creation
- directed follow edges
- creation of immutable snapshots

A `Snapshot` captures the entire follow graph at the moment it is created. Later mutations to the live `SocialNetwork` must not affect previously created snapshots.

### API Skeleton (illustrative; you may adjust internal fields but not behavior)

```python
class SocialNetwork:
    def __init__(self):
        """Create an empty network with no users and no follow edges."""
        ...

def add_user(self, user_id: str) -> None:
        """
        Register a new user.

Args:
            user_id: Unique string ID for the user.

Raises:
            ValueError: if the us

Accepted Answer

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