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Lesson 1 of 15

35. Design a Leaderboard (Top-K)

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The interview prompt
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35.1 What do you ask first?
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35.2 How big is this, really?
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35.3 What are the doors?
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35.4 What do we store?
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35.5 What does the whole system look like?
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35.6 How do sorted sets work for this?
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35.7 How do you handle real-time updates?
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35.8 How do you handle ties?
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35.9 How do you shard a leaderboard?
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35.10 What would you say in the last five minutes?
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Score yourself
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Variations to practise
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Sources
15
Code lab
35. Design a Leaderboard (Top-K)

The interview prompt

3 min read+10 XP

This chapter is a full interview round, played out step by step. The interviewer's lines are in quotes. At each step you'll see what a strong candidate says, the pushback the interviewer throws, and how to answer it. At the end, grade yourself with the rubric.

How to use this chapter

  1. Read only the prompt below. Then stop.
  2. Set a 45-minute timer and do the whole round yourself, on paper or a whiteboard, out loud. Use the framework from Chapter 1: READ, then Draw, Dive, Defend.
  3. Then read the walkthrough and compare. Score yourself with the rubric at the end.

The running example is a fantasy cricket contest during a big T20 final: users build teams of 11 real players before the match, and earn points as those players score runs and take wickets. The twist that makes it hard: one ball in the real match changes millions of users' scores at once.

The prompt

"Design the live leaderboard for a fantasy cricket contest. Millions of users join; during the match, everyone wants to see the top players and their own rank, updating as the match goes on."

Stop here. Start your timer.