Implementation details:
Email ownership: Map each email to the first account that owns it
Union on collision: If an email is already owned, union the accounts
Collect by root: After all unions, group emails by their root account
Sort and format: Each result entry is [name, sorted emails] Edge cases:
- Same name, different people (accounts don't merge unless they share an email)
- Single account (no merging needed)
- All accounts merge into one
Time: for unions where is average emails per account, plus for sorting. Space: for email map and result.