Linked User: Detect Accounts Linked by Shared Contact Information

Question

## Problem

You are given a list of user accounts. Each account has a user ID and a list of contact values (emails or phone numbers). Two accounts are "linked" if they share at least one contact value. Find all groups of linked accounts (connected components).

Return groups as lists of sorted user IDs, sorted by the smallest ID in each group.

```python
from typing import List

def linked_groups(accounts: List[List]) -> List[List[int]]:
    # accounts: [[user_id, contact1, contact2, ...], ...]
    # return: [[user_ids in group], ...] sorted
    pass
```

**Example:**
```
accounts = [
  [1, "a@x.com", "555-1234"],
  [2, "b@x.com"],
  [3, "a@x.com"],
  [4, "b@x.com", "555-9999"]
]

User 1 and 3 share "a@x.com" -> linked
User 2 and 4 share "b@x.com" -> linked

Output: [[1, 3], [2, 4]]
```

## Approach

Union-Find: map each contact value to the first user ID that owns it; union subsequent users with the same contact to that user.

## Follow-ups

1. What if a single user can be in multiple link chains — does Union-Find still work?
2. How would you update the grouping incrementally as new accounts are registered?
3. What privacy implications should you consider before building this feature into a production system?
4. How would you handle case-insensitive matching for email addresses?

Linked User: Detect Accounts Linked by Shared Contact Information

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