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Six worlds · start with Academy

World 1 · Kestrel Academy · Foundations

Academy.

One module after another. Each module has short sections: lessons teach, practice problems build the skill, and a checkpoint quiz ends the section. Everything is open: go in any order.

Academy progress0 of 25 modules done

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M3 · Hash House · Kestrel Pay

Hashing

Kestrel Pay's records hall lives here. You learn to trade memory for time, so that finding a key takes one step instead of a scan.

Section 1 · Inside a hash table0 / 50 of 5 steps done

Lesson · Hash table internals, next
Practice problem: The busiest payment shard, open
Practice problem: Growing a session table, open
Practice problem: Order codes that land in the same bucket, open
Checkpoint quiz + 2 problems, open
Next section: 2 · Seen-set and complement, open

Read the lesson ▶ Lecture

Module review · Hash House reviewbeing written

Browse everything · 74 lessons and problems, lectures, shorts, cheat sheets

LecturesWatch each lesson animated →ShortsOne idea in two minutes →Cheat sheetsEvery pattern on one page →

Lessons ready to learn

M3 · hashingHash table internalsExplain buckets, collisions, load factor and resizing.
M3 · hashingSeen-set and complementFind pairs and duplicates in one pass by remembering what you have seen.
M4 · two pointersOpposite endsMove pointers inward from both ends of a sorted sequence.
M4 · two pointersRead and write pointersFilter or compact an array in place with a read pointer and a write pointer.
M4 · two pointersThree-way partitionSplit values into less, equal and greater in one pass.
M4 · two pointersMerging sorted sequencesMerge two sorted lists in linear time.
M4 · two pointers3-sumCombine sorting and two pointers to find triples without duplicates.
M5 · sliding windowSliding window: fixed and variable windowsAnswer questions about contiguous stretches in one pass, with a window of fixed size or one that grows and shrinks.
M5 · sliding windowWindows with countsKeep a frequency map inside the window to compare stretches with a pattern.
M5 · sliding windowAt most K and exactly KFind the longest window with at most K of something, and count windows with exactly K.
M5 · sliding windowMinimum covering windowFind the shortest window that contains every required item.
s1 linked listsLinked listsPicture nodes and next pointers in memory, know what each operation costs, and build a linked list yourself.
a1 graph traversalBFS and DFS on a gridExplore a grid in rings with a queue for the fewest steps, or deep first with a stack to reach every cell, and build both yourself.

Practice problems

Not solved yetRepaint a region of map tileseasy
Not solved yetCount the rack blocks on a warehouse flooreasy
Not solved yetFewest moves to a pickup pointeasy
Not solved yetMinutes until a config reaches every servereasy
Not solved yetThe busiest payment shardeasy
Not solved yetHow long are the chains?medium
Not solved yetGrowing a session tablemedium
Not solved yetLookup cost in a chained tablehard
Not solved yetOrder codes that land in the same bucketmedium
Not solved yetFour ledgers that balancemedium
Not solved yetThe first replayed requesteasy
Not solved yetThe farthest matching pairmedium
Not solved yetA refund pair for a totaleasy
Not solved yetTrack pairs that fill whole slotsmedium
Not solved yetMerge two devices' message logseasy
Not solved yetDocuments in both posting listseasy
Not solved yetWhen the dock and the driver are both freemedium
Not solved yetMirror reference codeseasy
Not solved yetTwo parcels for the spare capacityeasy
Not solved yetGift pairs within the budgetmedium
Not solved yetSquared clock drifts in ordereasy
Not solved yetFewest cargo bikes for the roundmedium
Not solved yetCompact the log buffereasy
Not solved yetTrim repeated postingsmedium
Not solved yetCompress repeated scansmedium
Not solved yetClosest balance snapshots of two accountsmedium
Not solved yetThree products for a gift cardmedium
Not solved yetThree parcels closest to the pallet limitmedium
Not solved yetFour batches that net to zerohard
Not solved yetAd slot triples under the page budgetmedium
Not solved yetFirst use of each idempotency keyeasy
Not solved yetGroup parcels by priority classeasy
Not solved yetBids around the reserve pricemedium
Not solved yetTyped queries with backspacesmedium
Not solved yetThe k-th fastest pickupmedium
Not solved yetFee stretches within the allowancemedium
Not solved yetLongest segment covering at most two zonesmedium
Not solved yetStretches with exactly k clicksmedium
Not solved yetLongest healthy stretch with k re-runsmedium
Not solved yetSessions with exactly k categorieshard
Not solved yetBusiest stretch on the launch dashboardeasy
Not solved yetRepeated order inside k arrivalseasy
Not solved yetError bursts on the status pageeasy
Not solved yetFirst slow stretch in a surge zonemedium
Not solved yetLongest delivery run in one shiftmedium
Not solved yetLongest fresh listening sessionmedium
Not solved yetFewest hours to a revenue targetmedium
Not solved yetFewest parcels from both endshard
Not solved yetShortest log window with every required codemedium
Not solved yetShortest block to rebalance the build queuehard
Not solved yetShortest stretch where every member postsmedium
Not solved yetShortest browsing span with every wishlist itemhard
Not solved yetDistinct tracks in every k-play windoweasy
Not solved yetScrambled blocked terms in a descriptionmedium
Not solved yetWindows dominated by one topicmedium
Not solved yetReordered batch in a scan streammedium
Not solved yetBest bundle of distinct productsmedium
Not solved yetNewest first in a chat threadeasy
Not solved yetThe middle of a reply chaineasy
Not solved yetMerge two shards' log chainseasy
Not solved yetUnsend the k-th newest messageeasy

all problems →