Friends Invitation - Viral Referral System with Reward Tracking

Question

## Problem

Design a friends invitation and reward tracking system. When user A invites user B who signs up, A earns a reward. If B then invites C who signs up, A also earns an indirect reward (one level deep only). Implement:

- `invite(inviter_id, invitee_email)` - record invitation.
- `signup(email, user_id)` - register the user and trigger rewards.
- `get_rewards(user_id) -> int` - total reward credits earned.
- `get_invitees(user_id) -> List[str]` - list of successfully signed-up direct invitees.

```python
class InvitationSystem:
    def invite(self, inviter_id: str, invitee_email: str): ...
    def signup(self, email: str, user_id: str): ...
    def get_rewards(self, user_id: str) -> int: ...
    def get_invitees(self, user_id: str) -> List[str]: ...
```

**Example:**
```
sys.invite("u1", "bob@x.com")
sys.signup("bob@x.com", "u2")     # u1 earns 10 credits (direct)
sys.invite("u2", "carol@x.com")
sys.signup("carol@x.com", "u3")   # u2 earns 10, u1 earns 5 (indirect)
sys.get_rewards("u1") -> 15
```

## Follow-ups

1. How do you prevent reward farming (user inviting themselves with fake emails)?
2. Extend the reward chain to arbitrary depth with diminishing returns.
3. How would you idempotently handle duplicate `signup` calls for the same email?
4. How do you generate an invitation leaderboard (top inviters) efficiently?

Friends Invitation - Viral Referral System with Reward Tracking

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