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Counting, lookups and remembering what you have seen. Learn it first: Arrays, Strings, Hash Tables, Prefix Sums.

• Matching ReceiptsTwo Sumhash mapEasy
• Double-Scanned TicketsContains Duplicatehash mapEasy
• Same Letter TilesValid Anagramhash mapEasy
• The Missing Exam PaperMissing NumbermathEasy
• One-of-a-Kind BeadFirst Unique Character in a Stringhash mapEasy
• Circling the GarageContains Duplicate IIhash mapEasy
• Field Trip VoteMajority Elementhash mapEasy
• Comic Book FlipBest Time to Buy and Sell StockgreedyEasy
• Split the Night MarketFind Pivot Indexprefix sumsEasy
• Rotating Chore ChartRotate Arraytwo pointersMedium
• Jumble Puzzle BinsGroup Anagramshash mapMedium
• Bakery BestsellersTop K Frequent Elementshash mapMedium
• Power-Up MultipliersProduct of Array Except Selfprefix sumsMedium
• No-Repeat Garden BedsValid Sudokuhash mapMedium
• Longest Reading StreakLongest Consecutive Sequencehash mapHard
• Net Climb StretchesSubarray Sum Equals Kprefix sumsHard

Walk an array from both ends, or keep a moving window. Learn it first: Two Pointers, Sliding Window.

• Squared DeviationsSquares of a Sorted Arraytwo pointersEasy
• Mirror Phrase ContestValid Palindrometwo pointersEasy
• Merged Departure BoardRemove Duplicates from Sorted Arraytwo pointersEasy
• Bakery Tray GapsMove Zeroestwo pointersEasy
• Food Truck Best StretchMaximum Sum Subarray of Size Ksliding windowEasy
• Backdrop Between PolesContainer With Most Watertwo pointersMedium
• Neutral Charge Trios3Sumtwo pointersMedium
• Sushi Belt Without RepeatsLongest Substring Without Repeating Characterssliding windowMedium
• Shortest Steep ClimbMinimum Size Subarray Sumsliding windowMedium
• Adjacent Spice JarsPermutation in Stringsliding windowMedium
• Repaint the FenceLongest Repeating Character Replacementsliding windowMedium
• Market Street ErrandMinimum Window Substringsliding windowHard
• Rain on Brick TowersTrapping Rain Watertwo pointersHard

Pointer surgery and last-in, first-out thinking. Learn it first: Linked Lists, Stacks, Queues and Deques, Monotonic Stack.

• Reverse the PlaylistReverse Linked Listpointer reversalEasy
• Nesting Gift BoxesValid ParenthesesstackEasy
• Middle of the ParadeMiddle of the Linked Listtwo pointersEasy
• One Van, Two RoutesMerge Two Sorted Liststwo pointersEasy
• Vintage Calculator TapeEvaluate Reverse Polish NotationstackEasy
• Two-Spike Order RailImplement Queue using StacksdesignEasy
• Lowest Point on the TrailMin StackdesignMedium
• Closed Tram StopRemove Nth Node From End of Listtwo pointersMedium
• Symmetric Bead StrandPalindrome Linked Listtwo pointersMedium
• Stadium Gate TalliesAdd Two NumbersmathMedium
• Waiting for a Bigger WaveDaily Temperaturesmonotonic stackMedium
• Freezer Warmest StretchSliding Window MaximumqueueHard
• Mural on the Stepped WallLargest Rectangle in Histogrammonotonic stackHard

Halve the search space, or sort first and make the problem easy. Learn it first: Simple Sorts, Merge Sort, Quicksort, Heap Sort.

• Raffle Stub LookupBinary Searchbinary searchEasy
• New Dish on the MenuSearch Insert Positionbinary searchEasy
• Square Patio TilesSqrt(x)binary searchEasy
• Two-Heat Results BoardMerge Sorted Arraytwo pointersEasy
• Warehouse Bin GridSearch a 2D Matrixbinary searchEasy
• Postcode Stretch on the VanFind First and Last Position of Element in Sorted Arraybinary searchMedium
• Night Guard RoundSearch in Rotated Sorted Arraybinary searchMedium
• Photo Contest PodiumKth Largest Element in an ArraysortingMedium
• Biggest Number From CardsLargest NumbersortingMedium
• Book Club Reading PlanCapacity To Ship Packages Within D Daysbinary searchMedium
• How Messy Is the ShelfCount InversionssortingHard
• Two-Station MedianMedian of Two Sorted Arraysbinary searchHard

Recursion on trees, traversals and search trees. Learn it first: Binary Trees, Tree Traversals, Binary Search Trees, Balanced Trees.

• Generation AveragesAverage of Levels in Binary Treebreadth-first searchEasy
• How Deep the Cave GoesMaximum Depth of Binary Treedepth-first searchEasy
• Matching Phone MenusSame Treedepth-first searchEasy
• Iron-On Transfer FlipInvert Binary Treedepth-first searchEasy
• Will the Mobile Hang StraightSymmetric Treedepth-first searchEasy
• Adventure Book GoldPath Sumdepth-first searchEasy
• Acrobats From the WingsBinary Tree Right Side Viewbreadth-first searchMedium
• Longest Museum StrollDiameter of Binary Treedepth-first searchMedium
• Library Index AuditValidate Binary Search Treedepth-first searchMedium
• K-th Fastest FinisherKth Smallest Element in a BSTdepth-first searchMedium
• Closest Shared MentorLowest Common Ancestor of a Binary Treedepth-first searchMedium
• Rebuild the Escape Room CluesConstruct Binary Tree from Preorder and Inorder TraversalrecursionMedium
• Vineyard Tasting RouteBinary Tree Maximum Path Sumdepth-first searchHard

Grids, networks, dependencies and shortest paths. Learn it first: Graph Basics, Breadth-First Search, Depth-First Search, Topological Sort.

• Recolor the Cross-StitchFlood Filldepth-first searchEasy
• Parking Lot PuddlesNumber of Islandsdepth-first searchEasy
• Corn Maze ShortcutShortest Path in a Mazebreadth-first searchEasy
• Loose Pieces on the TableNumber of Connected Components in an Undirected Graphunion-findEasy
• Two Buses for the TripIs Graph Bipartite?breadth-first searchEasy
• Aphids in the GreenhouseRotting Orangesbreadth-first searchMedium
• One Cable Too ManyRedundant Connectionunion-findMedium
• Flat-Pack Assembly OrderCourse Schedule IItopological sortMedium
• Staff Alert RelayNetwork Delay Timeshortest pathMedium
• Wiring the CampsitesMin Cost to Connect All PointsgreedyMedium
• Parcel With Few HandoffsCheapest Flights Within K Stopsshortest pathHard
• Marquee Letter SwapsWord Ladderbreadth-first searchHard

Try a choice, explore, undo, try the next. Learn it first: Recursion, Backtracking, Divide and Conquer.

• Moving the Wedding CakeTower of HanoirecursionEasy
• Rabbit Counter WraparoundPow(x, n) / Modular ExponentiationrecursionEasy
• Burger Truck Test OrdersSubsetsbacktrackingEasy
• Relay Practice LineupsPermutationsbacktrackingEasy
• Postcard Mountain SkylinesGenerate ParenthesesbacktrackingMedium
• Exact Postage StampsCombination SumbacktrackingMedium
• Smudged Router LabelRestore IP AddressesbacktrackingMedium
• Café Letter BoardWord SearchbacktrackingMedium
• Festival Train SplitPalindrome PartitioningbacktrackingMedium
• Gallery Ceiling SensorsN-Queens IIbacktrackingHard
• Puzzle Page Mini GridSudoku Solver (6 × 6)backtrackingHard

Make the locally best move, and know when that is safe. Learn it first: Greedy Algorithms, Intervals.

• Coat Drive HandoutAssign CookiesgreedyEasy
• Ferry Fare ChangeLemonade ChangegreedyEasy
• Resurfaced Road StretchesMerge IntervalsintervalsEasy
• Relay Towers Down the ValleyJump GamegreedyEasy
• Studio Booking SheetInsert IntervalintervalsMedium
• Parcel Line Into TrucksPartition LabelsgreedyMedium
• Shared Telescope NightNon-overlapping IntervalsintervalsMedium
• Laundromat Machine CountMeeting Rooms IIintervalsMedium
• Hut-to-Hut TrekJump Game IIgreedyMedium
• Ring Line Tram BatteryGas StationgreedyHard
• Gym Sets With RestTask SchedulergreedyHard

Define the subproblem, then fill the table. Learn it first: Introduction to Dynamic Programming, 1D Dynamic Programming, DP on Grids, Knapsack.

• Stepping Stones Across the PondClimbing Stairsdynamic programmingEasy
• Sunny Orchard RowHouse Robberdynamic programmingEasy
• Arcade Tile RunMaximum Subarray (Kadane's Algorithm)dynamic programmingEasy
• Roadworks on the GridUnique Paths IIdynamic programmingEasy
• Snowplow to the RoadMinimum Path Sumdynamic programmingEasy
• Cookie Box OrderCoin Change (fewest coins)dynamic programmingMedium
• Vending Machine Coin MixesCoin Change II (number of combinations)dynamic programmingMedium
• Secret Number NotesDecode Waysdynamic programmingMedium
• Squashed HashtagWord Breakdynamic programmingMedium
• Typing Class ProgressLongest Increasing Subsequencedynamic programmingMedium
• Shared Dance MovesLongest Common Subsequencedynamic programmingMedium
• Busking Set List0/1 Knapsackdynamic programmingMedium
• Sneaker Resale With a Shipping DayBest Time to Buy and Sell Stock with Cooldowndynamic programmingHard
• Spelling Test FixesEdit Distance (Levenshtein Distance)dynamic programmingHard

Specialised structures and low-level tricks. Learn it first: Heaps and Priority Queues, Tries, Bit Manipulation, String Matching.

• Knockout Bracket CheckPower of Twobit manipulationEasy
• Last Car in the GarageSingle Numberbit manipulationEasy
• Binary Counter ExhibitCounting Bitsbit manipulationEasy
• Upside-Down Light StripReverse Bitsbit manipulationEasy
• Snowball ShowdownLast Stone WeightheapEasy
• Quiz League Final CutoffKth Largest Element in a StreamheapMedium
• Drone Nearest Drop-OffsK Closest Points to OriginheapMedium
• Glowing Letter TilesImplement Trie (Prefix Tree)trieMedium
• Town Baby Name ChartTop K Frequent WordsheapMedium
• Garden Centre Plant FinderSearch Suggestions SystemtrieMedium
• Wedding Camera TimelineMerge k Sorted ListsheapHard
• Live Median House PriceFind Median from Data StreamheapHard

Build the core pieces of machine learning with plain code. Learn it first: How Models Learn, Regression and Classification, Overfitting and Evaluation, Decision Trees and Ensembles.

• Ice Cream Forecast CheckMean Squared ErrormathEasy
• Bookshop Blurb MatchCosine Similaritylinear algebraEasy
• Trail Guide SnippetsOverlapping Text Chunkingsliding windowEasy
• Bruised Apple DetectorPrecision, Recall and F1 ScoremathEasy
• Delivery Time NudgeOne Step of Gradient Descent for Linear Regressiongradient descentEasy
• Storyteller Word OddsSoftmax with TemperatureprobabilityMedium
• Unlabelled Seed Packetk-Nearest Neighbours ClassificationsortingMedium
• Pronoun Looks BackScaled Dot-Product Attention Weightslinear algebraMedium
• Sign Shop Letter BlocksGreedy Longest-Match TokenizergreedyMedium
• Festival Charging Cartsk-Means ClusteringsimulationMedium
• Knitting Pattern ShorthandByte Pair Encoding (BPE) Mergeshash mapHard