Show Streak: Find a User's Longest Consecutive Daily Viewing Streak

Question

## Problem

Given a list of dates on which a user watched at least one episode, find the length of their longest consecutive daily viewing streak.

Dates are given as strings in `YYYY-MM-DD` format, possibly unsorted and with duplicates.

```python
from typing import List

def longest_streak(watch_dates: List[str]) -> int:
    # return the length of the longest consecutive day streak
    pass
```

**Example:**
```
Input:  ["2024-01-03", "2024-01-01", "2024-01-02", "2024-01-01", "2024-01-05"]
Output: 3
# Streak: Jan 1 -> Jan 2 -> Jan 3. Jan 5 is isolated.
```

## Approach

Convert to a set of date objects. For each date that is NOT preceded by (date - 1 day), walk forward counting consecutive days. Track the maximum.

Time: O(n), Space: O(n).

## Follow-ups

1. How would you modify this to find the current (ongoing) streak as of today's date?
2. What if a "streak" allows one missed day as a grace period? How does the algorithm change?
3. How would you compute this across 100 million users daily using a distributed framework like Spark?
4. If users are in different time zones, how do you define "same day" consistently?

Show Streak: Find a User's Longest Consecutive Daily Viewing Streak

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