Longest Streak of Consecutive Login Days by a Single User
Company: Amazon
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Technical Screen
You are given login records, each a user id and the calendar date of one login. Return the length of the longest streak of consecutive days on which a single user logged in. A streak ends as soon as that user's next login date is more than one day after the previous one; counting then starts again.
The candidate's report does not say whether the answer is reported per user or overall; this version returns the longest streak achieved by any single user.
### Function Signature
```python
def longest_login_streak(logins: list[list[str]]) -> int:
```
### Rules
- Each record is `[user_id, date]`, where `date` is a valid Gregorian calendar date in `YYYY-MM-DD` format.
- Records are in no particular order, and a user may log in several times on the same day; those logins count as one day.
- A streak is a maximal run of consecutive calendar days on each of which one user logged in at least once. Consecutive days cross month and year boundaries normally, and February 29 is a real day in leap years.
- A streak belongs to one user: logins by other users never extend or break it.
- Return the length, in days, of the longest streak over all users.
### Constraints
- `1 <= len(logins) <= 10^5`
- `user_id` has 1 to 20 characters, each a lowercase English letter or a digit.
- Every date lies between `2000-01-01` and `2099-12-31`.
### Examples
**Example 1**
```text
Input: logins = [["u1", "2026-05-03"], ["u2", "2026-05-03"], ["u1", "2026-05-04"], ["u1", "2026-05-04"], ["u1", "2026-05-06"], ["u2", "2026-05-04"], ["u2", "2026-05-05"]]
Output: 3
```
User `u1` logged in on May 3, May 4 (twice) and May 6, giving streaks of 2 and 1 days. User `u2` logged in on May 3, 4 and 5, a streak of 3 days.
**Example 2**
```text
Input: logins = [["a", "2024-03-01"], ["a", "2024-02-28"], ["a", "2024-02-29"], ["b", "2024-03-02"]]
Output: 3
```
User `a` logged in on February 28, February 29 (2024 is a leap year) and March 1, a streak of 3 days. User `b`'s login on March 2 does not extend `a`'s streak.
**Example 3**
```text
Input: logins = [["z", "2025-12-31"], ["z", "2026-01-02"]]
Output: 1
```
January 1 is missing, so the two logins form two separate streaks of 1 day.
Overview: Given unordered login records of user ids and calendar dates, return the longest run of consecutive days on which a single user logged in, counting repeat logins on the same day once. Tests grouping by user, date arithmetic across month, year and leap-day boundaries, and counting runs that reset after a gap.
