Design a referral leaderboard with chain-based counts

Design and implement a function that generates a referral leaderboard for a platform, given two equal-length arrays rh_users and new_users representing referrals in chronological order (rh_users[i] referred new_users[i]). A user is considered to have referred everyone downstream in their referral chain (e.g., for A -> B -> C -> D, A referred B, C, and D; B referred C and D; C referred D). Requirements:

• Referral rules:
  • A user can be referred at most once.
  • Once a user is on the platform, they cannot be referred by others later.
  • The input pairs appear in the order they were created.
• Leaderboard rules:
  • Include only users with referral count >= 1.
  • Return at most the top 3 users.
  • Sort by descending referral count; break ties by username in ascending lexicographic order.
• Input:
  • rh_users: string[] of referrers.
  • new_users: string[] of referred users, aligned by index with rh_users.
• Output:
  • string[] of up to 3 entries in the form " <user> <count> ". Example:
• rh_users = ["A", "B", "C"], new_users = ["B", "C", "D"].
• Output: ["A 3", "B 2", "C 1"]. Discuss your data structures, time and space complexity, and provide runnable code in the language of your choice.