DSA & System Design

Interview prep, at your pace

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Then run it yourself.

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System design A request's journey

A request's journeyA request travels from the browser through the CDN and load balancer to a service, which reads Redis and falls back to Postgres on a miss.BrowserCDNLoad balancerService ×3RedisPostgresQueue

DSA Two pointers find a pair

nums (6)
1
3
4
6
8
11
▲ left
▲ right

The array is sorted. Put left on the smallest value (index 0) and right on the largest (index 5).

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Break things on purpose

Push a slider until something gives. You learn the limits faster when you find them yourself.

System design simulators

DSA labs

Shorts

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A hook, three points and a line to say in the interview. Swipe for the next card, scroll down to go deeper.

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System design · Requirements

Cut the scope, then put a number on every requirement

  • Name the 3 core actions as verbs. For Twitter: post, follow, read the timeline. Everything else hangs off these.
  • Cut features out loud: "I'll leave out DMs, search and ads." Cutting is a sign of seniority, not laziness.
  • Give every non-functional requirement a number: latency, availability, consistency, durability and scale.
Read this short DSA · Hashing

Replace the inner search with one O(1) lookup

  • Spot it: you keep asking "have I seen this before?", or you need how many times something occurs.
  • Spot it: each element needs a partner such as target - x, or items that share a property must be grouped.
  • Store what each element tells you as you pass it: presence in a Set, or a count, index or group in a Map.
Read this short Building blocks · Bloom filter

A Bloom filter answers "definitely not" in a few bytes per item

  • A bit array of m bits plus k hash functions. Adding a key sets the k bits its hashes point to.
  • To check a key, read its k bits. Any bit is 0: definitely not present. All are 1: maybe present.
  • No false negatives, ever. False positives happen when other keys happened to set all k bits.
Read this short