Hash maps, sets, and how caches use them
By the end of today you can explain what hashing actually does, recognise the class of problem where storing what you have already seen collapses a nested loop, and say why a cache is a hash map with a memory limit.
YesterdayOn Day 14 you collapsed a nested loop using two pointers, which needed sorted data. Today you collapse one without that requirement.
TomorrowTomorrow, sets apply the same hashing idea to membership and uniqueness.
Why this matters
The hash map is the single most useful data structure in practical engineering. The same idea appears as a database index on Day 43, a cache on Day 60, and a vector store lookup on Day 74.
- Hashing
- The hash map pattern
- Collisions
- Cache intuition
- Set algebra
- Membership testing
- Deduplication
- Recognising a set problem
Learn it
80 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
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Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
5 hand-picked resources
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Build it
50 minSolve two-sum on an unsorted list twice: nested loops, then one pass with a dict. Then write a function that finds the first non-repeating character in a string, and one that groups a list of words by their sorted letters. Time the two-sum versions at 10,000 items.
Recall it
25 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
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Mastery tracking
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Recap
- 01Hashing turns a key into a position, so lookup is direct rather than a search
- 02Collisions are inevitable and handled internally
- 03Remembering what you have seen collapses many nested loops into one pass
- 04A cache is a hash map that is allowed to forget
Your progress
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