Unwatched Episode: Find the First Episode a User Has Not Seen in a Series

Question

## Problem

A user has a watch history represented as a set of `(series_id, season, episode)` tuples. Given a series with `S` seasons and `E[s]` episodes per season, return the first unwatched episode in order, or `None` if all episodes are watched.

Episodes are ordered: S1E1, S1E2, ..., S1E_last, S2E1, ...

```python
from typing import Set, List, Optional, Tuple

def first_unwatched(
    series_id: str,
    episodes_per_season: List[int],
    watched: Set[Tuple[str, int, int]]
) -> Optional[Tuple[int, int]]:
    # return (season, episode) of first unwatched, or None
    pass
```

**Example:**
```
series_id = "SHOW1"
episodes_per_season = [3, 2]   # S1: 3 eps, S2: 2 eps
watched = {("SHOW1",1,1), ("SHOW1",1,2), ("SHOW1",1,3), ("SHOW1",2,1)}

Output: (2, 2)
# All of S1 and S2E1 are watched; S2E2 is first unwatched.
```

## Follow-ups

1. What if "watched" means the user watched at least 80% of an episode — how does the data model change?
2. How would you expose this as a REST API endpoint, and what caching strategy would you apply?
3. If the series is still airing and new episodes are added, how do you invalidate stale "continue watching" pointers?
4. How would you handle the edge case where watched contains entries for episodes that don't exist in the catalog?

Unwatched Episode: Find the First Episode a User Has Not Seen in a Series

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