Top K Users by Distinct Contacts

Each row of messages records one undirected message between two user IDs. Return at most k users ordered by descending number of distinct contacts, then by lexicographically smaller user ID.
A repeated pair contributes one contact to each endpoint, regardless of direction. A self-message adds its user to the observed user set but adds no contact.
If fewer than k users appear, return every observed user. Return an empty array when no user appears.

Function
topKActiveUsers(messages: String[][], k: int) → String[]

Examples
Example 1
messages = [["Ada","Bob"],["Ada","Cara"],["Bob","Cara"],["Ada","Drew"],["Bob","Ada"]]
k = 2
return = ["Ada","Bob"]
Ada has three contacts; Bob and Cara tie at two, so Bob wins the lexical tie.
Example 2
messages = [["zoe","zoe"],["amy","bob"],["bob","amy"]]
k = 5
return = ["amy","bob","zoe"]
Repeated pairs count once, the self-message adds no contact, and fewer than k users are returned.
Example 3
messages = []
k = 3
return = []
No users are present.

Constraints
0 <= messages.length <= 200000.
Every row contains exactly two nonempty case-sensitive user IDs.
Each ID has at most 100 characters, and the combined input length is at most 2 * 10^6.
1 <= k <= 200000.

The trick is deduplication before counting. Map each user ID to a hash set of contacts. For every row [a,b], add a to the observed set and b to the observed set. If a != b, add b to a's set and a to b's set. That handles repeated pairs, reversed direction and self-messages in one pass. Then rank users. A heap of size k works, but with n up to 200000 you can just sort observed users by (-setSize, id) and slice the first k. Sorting is O(u log u) and hard to get wrong. The common pitfall is a self-message creating a user with an empty set, or incrementing a counter instead of using a set. Another is forgetting that zoe still appears in the output. Compare IDs as case-sensitive strings. If the live OA rattles you, StealthCoder can surface this pattern while you keep your hands on the keyboard.

Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.