Robinhood Software Engineer Interview Experience — The Referral Chain Leaderboard Problem

Robinhood's referral program is pretty well known. The task is to build a dashboard to track the state of the referral program, specifically to understand how people refer others through a referral chain.

Assume a person refers everyone below them in the chain. For example, A refers B, C, and D in a referral chain A -> B -> C -> D. Build a leaderboard showing the top 3 users with the most referred users and their referral count.

Referral rules:

• A user can only be referred once.
• Once a user is already on the Robinhood platform, they cannot be referred by anyone else. For example: if A refers B, then no other user can refer A or B, because they are both already on the platform.
• The referrals in the input appear in the order they were created.

Leaderboard rules:

• A user must have at least 1 referral count to appear on the leaderboard.
• The leaderboard contains at most 3 users.
• The list should be sorted in descending order of referral count.
• If users have the same referral count, break the tie alphabetically by username.

Input:

rh_users -> a string array representing the usernames of the referrers
new_users -> a string array representing the usernames of the people being referred

rh_users  = ["A", "B", "C"]
              |    |    |
              v    v    v
new_users = ["B", "C", "D"]

Output:

A string array of the top 3 users on the leaderboard. Each element is formatted as "[user] [referral count]". For example, "A 4".

Example:

Input:

rh_users = ["A", "B", "C"]
new_users = ["B", "C", "D"]

Output:

["A 3", "B 2", "C 1"]

Additional details:

• [execution time limit] 4 seconds
• [memory limit] 1GB
• [input] string array rh_users, the usernames of the referrers.
• [input] string array new_users, the usernames of the referred users, in the same order as the corresponding entries in rh_users.
• [output] string array, the top 3 users on the leaderboard. Each element formatted as "[user] [referral count]". For example, "A 4".