Solve a lot. Mostly a little above your level.

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFWatermeloncoreyour first submission800
• CFWay Too Long Wordscore800
• CFTeam800
• CFBit++800
• CFTheatre Squareneeds a 64-bit integer1000
• CSESWeird Algorithm

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFCandies and Two Sisters800
• CFYet Another Two Integers Problem800
• CFMinimal Squarecore800
• CFSum of Round Numberscorework the examples by hand first800
• CFCards for Friends800
• CFOrdinary Numbers800
• CFOdd Divisor900
• CFExpression1000
• CFSame Parity Summandsa little above the band1200

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFSumcoremultiple test cases800
• CFA+B Again?800
• CFPlus or Minus800
• CFDivision?800
• CFElephant800
• CFSoldier and Bananascorea sum that grows800
• CSESIncreasing Array
• CSESBit Stringsmodulo 10^9+7

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFDivisibility Problem800
• CFMaximum GCDcoredo not try every pair800
• CFSum of Cubes1100
• CFK-th Not Divisible by ncorek is up to 10^91200
• CSESMissing Number
• CSESTrailing Zerosn up to 10^9
• LCPower of TwoEasy

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFFloor Numberceiling division800
• CFRestoring Three Numbers800
• CFLCM Problem800
• CFGame with Integers800
• CFEven Oddscore900
• LCPalindrome NumberEasy
• LCAdd DigitsEasy

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFBeautiful Matrixcore800
• CFTram800
• LCFizz BuzzEasy
• LCRobot Return to OriginEasy
• CFStones on the Table800
• CFPetya and Stringscore800
• CFWord Capitalization800
• CFFootball900
• CFString Taskcore1000
• CFChat room1000
• CFXenia and Ringroadsimulate, watch the wrap-around1000
• LCSpiral MatrixMedium
• LCRotate ImageMedium
• LCGame of LifeMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFHelpful Maths800
• CFMedium Number800
• CFTwinscore900
• CFGravity Flip900
• CFDragonscore1000
• CFInteresting drinkcoresort once, then search for each query1100
• CFWorms1200
• CFVanya and Lanterns1200
• CFQueries about less or equal elementsa little above the band1300
• CSESDistinct Numbers
• LCSearch Insert PositionEasy

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFPangram800
• CFICPC Balloons800
• CFWord Gamecore800
• CFAdvantage800
• CFA and B and Compilation Errors1100
• CFSame Differencescorecount with a map1200
• CFMatryoshkas1200
• CSESConcert Ticketsneeds a sorted multiset
• LCTwo SumcoreEasy
• LCContains DuplicateEasy
• LCValid AnagramEasy

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFBoy or Girlcoredistinct letters with a frequency array800
• CFDo Not Be Distracted!corefrequency array plus one pass800
• CFShort Substrings800
• CFReverse a Substringlook at adjacent pairs1000
• LCMove Zeroesthe in-place write pointerEasy
• LCValid PalindromeEasy
• LCIs SubsequenceEasy

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCRange Sum Query - ImmutableEasy
• CSESStatic Range Sum Queries
• CFIlya and Queriescoreprefix counts over adjacent pairs1100
• CFThree Parts of the Arrayprefix sums plus two pointers1200
• LCRange Sum Query 2D - ImmutableMedium
• CFKaren and Coffeecoredifference array, then prefix counts1400
• CFLittle Girl and Maximum Sumdifference array, then sort and match1500
• LCSubarray Sum Equals KMedium
• LCCorporate Flight BookingsMedium
• CSESSubarray Sums II
• CFZero-Sum Prefixes1600
• CFNumber of Wayscoresplit into three equal-sum parts1700

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCGroup Anagramsa sorted word is a good map keyMedium
• CFRegistration Systemmap from a name to how often it was used1300
• CFYarik and Musical Notesfind a pair-equality rewrite, then count1300
• LCLongest Consecutive Sequencea set, and only start from sequence beginningsMedium
• CFWinnertwo passes with a map1500

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFFavorite Sequence800
• CFSereja and Dima800
• CFEating Candiescoreleft and right pointers with running sums1100
• CFBerSU Ballcoresort, then match from both lists1200
• LCTwo Sum II - Input Array Is SortedMedium
• CSESSum of Two Valueskeep the original indices when you sort
• CSESFerris Wheel
• CFNumber of Pairscorecount pairs with sum in a range1300
• LCContainer With Most WaterMedium
• CFPair of Topics1400
• LC3SumMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFBlack and White Stripecorefixed window1000
• CFSki Resortcount segments by their length1000
• LCMaximum Average Subarray IEasy
• CSESPlaylistlongest window with all values distinct
• CFTernary Stringshortest window containing 1, 2 and 31200
• CFBinary Dequecorelongest window with a given sum1200
• LCLongest Substring Without Repeating CharacterscoreMedium
• LCMinimum Size Subarray SumMedium
• CFBookslongest window with sum at most t1400
• CFVasya and Stringcorewindow with at most k letters to change1500
• LCPermutation in StringMedium
• LCSliding Window MaximumHard

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCBinary SearchEasy
• LCSqrt(x)last index where mid * mid <= xEasy
• CFEating Queriesprefix sums are sorted, so search them1100
• LCFind First and Last Position of Element in Sorted ArrayMedium
• CFT-primes1300
• LCSearch in Rotated Sorted ArrayMedium
• LCFind Peak Elementbinary search without sorted dataMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCKoko Eating BananasMedium
• CFBuilding an Aquariumcorefind the largest height whose water fits1100
• CFPoisoned Dagger1200
• LCCapacity To Ship Packages Within D DayscoreMedium
• CSESFactory Machines
• CFMaximum Mediancoremaximise the median under an operation budget1400
• LCMagnetic Force Between Two Ballsmaximise the minimum distanceMedium
• CFMagic Powder - 21500
• CSESArray Division
• CFHamburgers1600
• CFPresentthe check uses a difference array1700

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFMove Bracketscorebalance counter, the stack idea without the stack1000
• LCValid ParenthesescoreEasy
• LCRemove All Adjacent Duplicates In Stringthe string itself is the stackEasy
• LCNext Greater Element IEasy
• CFRegular Bracket Sequencelongest regular subsequence, count matched pairs1400
• LCMin Stackstore the running minimum next to each valueMedium
• LCDaily Temperaturescorenext greater element, answer is the distanceMedium
• CFQueuefarthest smaller element to the right1500
• LCLargest Rectangle in Histogramnearest smaller on both sidesHard
• CFMike and Feetstretch; span in which each bar is the minimum1900

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCFibonacci NumberEasy
• LCMaximum Depth of Binary Treethe answer for a node comes from its two childrenEasy
• LCClimbing Stairsrecursion, then add a cacheEasy
• LCMerge Two Sorted ListsEasy
• LCReverse Linked Listdo it both recursively and with a loopEasy
• CFGold Rushcoretry the split, recurse on both parts1000
• CFTransformation: from A to Bwork backwards from b1000
• LCPow(x, n)corehalve the exponentMedium
• CFPermutation Transformationcoresplit the range at the maximum1200
• CFMasha and a Beautiful Treerecurse on halves of the array1300
• CFa-Good Stringstretch; two choices per level1500

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFPetr and a Combination Lockcoretry both directions for every rotation1200
• LCSubsetscoreMedium
• LCPermutationscoreMedium
• CFDreamoon and WiFitry every way to fill the unknown commands1300
• LCCombinationschoose k of n, prune when too few numbers remainMedium
• CFPreparing Olympiadevery subset of at most 15 problems1400
• LCGenerate Parenthesesprune when closers would exceed openersMedium
• LCCombination SumMedium
• LCWord Searchmark a cell, explore, unmarkMedium
• CSESChessboard and QueenscoreN-Queens with blocked squares
• LCN-QueensHard

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFAnd Then There Were Kcorelook at the highest set bit of n800
• LCSingle NumberXOR cancels pairsEasy
• LCNumber of 1 BitsEasy
• LCCounting Bitspopcount of every number up to nEasy
• CFRaising Bacteriacorethe answer is a popcount1000
• CFRoof Constructionsplit at the largest power of two below n1000
• LCMissing NumberEasy
• CFRock and Levercoregroup numbers by highest set bit1200
• CSESApple Divisiontry every subset with a mask
• CFMaximal ANDdecide bits from the highest to the lowest1300
• CFAGAGA XOOORRRXOR of all elements decides the number of parts1500

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFEhAb AnD gCd800
• CFBalanced Arraycorethe worked example in this lesson800
• CFMake it Beautiful800
• CSESPermutations
• CFDifferent Divisors1000
• CFChallenging Cliffscoresort, then place the closest pair at the two ends1200
• CFOdd Sum Segments1200
• CFSegments1300
• CFPhoenix and Beauty1400

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCAssign CookiesEasy
• CSESMovie Festivalcorethe activity selection problem from this lesson
• CSESTasks and Deadlines
• LCNon-overlapping IntervalsMedium
• CFNested Segments1500

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFPoints in Segments800
• CFFree Cashthe largest number of customers at the same minute1000
• CSESRestaurant Customerscoremaximum overlap, the template of this lesson
• LCMerge IntervalscoreMedium
• LCCar PoolingMedium
• LCInsert IntervalMedium
• LCDivide Intervals Into Minimum Number of GroupsMedium
• CFCovered Points Countcorehuge coordinates, events in a map1700
• CFToo Many Segments (easy version)1800

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFAlmost Prime900
• CFLucky Division1000
• LCCount PrimesMedium
• CFBuying Shovels1300
• CFProduct of Three Numbers1300
• CFk-th divisor1400
• CFFadi and LCMcoregcd, lcm and divisors together1400
• CFRow GCDcoregcd(a, b) = gcd(a, b - a)1600
• CSESCounting Divisors

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFModular Exponentiation900
• CSESExponentiation
• CFAND 0, Sum Bigcorethe answer is k to the power n, modulo 10^9 + 71200
• CFDivisible Pairs1300
• CSESExponentiation II
• CFAND Sequencescorecount with factorials, take the result modulo p1400
• CSESBinomial Coefficients
• CFAdvertising AgencycorenCr modulo a prime1600

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCPascal's TriangleEasy
• CFPasha and Stick1000
• LCUnique PathsMedium
• CFForming Trianglescoregroup by length, then choose with nCr1200
• CFRandom Teams1300
• CFTwo Round Dances1300
• CFK-th Beautiful String1300
• CFBall in Berlandcoretotal pairs minus bad ones, an inclusion-exclusion step1400

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCLast Stone WeightEasy
• CFSnacktowera heap of what has arrived, release the largest when it is due1100
• LCKth Largest Element in an ArraycoreMedium
• LCTop K Frequent ElementsMedium
• CFEpic Transformationcorealways pair the two most frequent values1400
• LCMerge k Sorted ListscoreHard
• CSESRoom Allocationheap of room end times
• CFConstructing the Array1600
• CFHeap Operations1600
• CFPotions (Hard Version)coreundo the worst past choice with a max-heap1600
• LCFind Median from Data Streamtwo heapsHard

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDiameter of Binary TreeEasy
• CSESSubordinatescoresubtree sizes, exactly the second half of this lesson
• CSESTree Diameter
• CFBegginer's Zelda1100
• CFChristmas Spruce1200
• CFApple Tree1200
• CFWhite-Black Balanced Subtreescorecount something inside every subtree1300
• CFQueen1400
• CFKefa and ParkcoreDFS that carries a running count down the path1500
• CFCut 'em all!1500

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFNew Year Transportation1000
• CFThe Lakescorecomponents on a grid, the simplest real use of DFS or BFS1100
• LCNumber of IslandsMedium
• CFIce Skatingcomponents where "connected" needs one observation1200
• CSESMessage RoutecoreBFS plus printing the path
• LCIs Graph Bipartite?Medium
• CFCyclic Components1500
• CFNP-Hard Problemcore2-colouring with a twist in the output1500
• CFValid BFS?what exactly can a BFS order look like1700
• CFSolve The Mazegrid reachability with a greedy wall placement1700

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCCourse ScheduleMedium
• LCCourse Schedule IIMedium
• CSESCourse Schedulecoreprint the order, or IMPOSSIBLE on a cycle
• LCFind Eventual Safe Statesrun Kahn on the reversed graphMedium
• CSESLongest Flight RouteDP over a DAG with a path to print
• CFFox And Namesbuild the letter order from adjacent names, then sort1600
• CFSubstringcoreDP on a DAG for every letter, a cycle means infinity1700
• CFBooklongest chain with a twist on the cost of each edge1800
• CFDirecting Edgescoreorder the directed edges first, then orient the rest by that order2000
• CFGourmet choicemerge equal items with DSU, then longest path on layers2000

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCNetwork Delay TimeMedium
• CSESShortest Routes Icoreplain Dijkstra, the first one to write from memory
• CSESShortest Routes IIall pairs, Floyd-Warshall
• LCCheapest Flights Within K StopsBellman-Ford with a limited number of roundsMedium
• CSESCycle Findingnegative cycle with Bellman-Ford, and print it
• CFGreg and GraphFloyd-Warshall, read the statement for the order of k1700
• CFLabyrinthcore0-1 BFS on a grid1800
• CFBicycles1800
• CFRendez-vous de Marian et RobinDijkstra with a state (node, has a horse)1800
• CFDijkstra?corethe classic, with path printing1900
• CFPaths and Trees2000

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCNumber of ProvincesMedium
• LCRedundant Connectionthe union that returns false is the extra edgeMedium
• CSESRoad Constructioncorethis lesson's template, almost word for word
• CFDZY Loves Chemistrythe answer depends only on the number of components1400
• CFNews Distributioncorecomponent sizes1400
• CFLearning Languagesone corner case when nobody knows any language1400
• CFKnight Tournamentcorethe "next free index" use of DSU1500
• CFBear and Friendship Condition1500
• CFThe Number of ImpostersDSU with a parity (same or different) on each link1700
• CFRoads not only in Berlandfind the useless edges and reuse them to connect the components1900

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCMin Cost to Connect All Pointsa complete graph, so Prim without a heap works wellMedium
• CSESRoad Reparationcorethe template, plus printing IMPOSSIBLE
• CFRoad Reformcorebuild the MST, then think about which edge is closest to the target1800
• CFShichikuji and Power Gridcoreadd a virtual node so that "build a station" becomes "take an edge"1900
• CFMake It Connectedonly a few edges can matter1900
• CF0-1 MSTthe weight-0 edges are the complement of the input1900
• CFGCD and MST2000
• CFMST Unificationequal-weight edges, process them in groups2100

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCMin Cost Climbing StairsEasy
• CFFilling Shapesfind the recurrence on small n, then simplify it1000
• CFCut Ribboncorethe worked example below1300
• CFVacationscorestate = (day, what you did yesterday)1400
• LCMinimum Cost For Ticketscorestate = day indexMedium
• CFFlowersstretch; DP plus prefix sums for range queries1700

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCHouse Robbercoretake or skipMedium
• CFBasketball Exercisetwo rows, never take two neighbours1400
• LCCoin ChangecoreMedium
• LCCoin Change IIcount combinations; loop order mattersMedium
• LCDecode WaysMedium
• LCJump Game IIDP first, then the greedy O(n)Medium
• CFBlock Sequencedp from the right end1500
• CFBoredomcorecount values, then it is House Robber1500
• CFk-Treeways to reach sum n; add a flag for the second condition1600

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFCoin Rowsa grid with two rows; prefix sums are enough, but think of it as a path1300
• CSESGrid Pathsthe DP version, count paths around traps
• LCMinimum Path SumMedium
• LCLongest Common SubsequencecoreMedium
• LCEdit DistancecoreMedium
• CSESEdit Distance
• CFHard problemcoretwo states per string - reversed or not1600
• CFZero Pathstretch; keep the minimum and maximum sum per cell1700
• LCLongest Palindromic SubsequenceLCS of a string with its reverseMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCPartition Equal Subset Sumcoresubset sum with target = total / 2Medium
• CSESBook Shopcorethe plain 0/1 knapsack
• LCLast Stone Weight IIsplit into two groups with the smallest differenceMedium
• LCTarget Sumcount subsets after a rewriteMedium
• LCOnes and Zeroesknapsack with two capacitiesMedium
• CFBaby Ehab Partitions Againcoresubset sum, then remove one element1700
• CFWriting Codecounting variant with an unbounded flavour1800
• CFModulo Sumstretch; subset sum on remainders, with a pigeonhole shortcut1900

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCLongest Increasing SubsequencecoreMedium
• CSESIncreasing Subsequencethe plain O(n log n) version with n up to 2*10^5
• CFBubble Sort Graphthe answer is an LIS of the permutation1500
• LCMaximum Length of Pair Chainsort, then LIS-styleMedium
• CFConsecutive Subsequencecorea hash map instead of a binary search1700
• CFMysterious PresentcoreLIS on pairs, with reconstruction1700
• LCNumber of Longest Increasing Subsequencekeep a count next to each lengthMedium
• LCRussian Doll Envelopessort by width ascending, height descending, then LIS on heightHard
• CFSubsequencesstretch; count increasing subsequences of a given length with a Fenwick tree1900

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCHouse Robber IIIcorethe independent-set DP of this lessonMedium
• LCBinary Tree Maximum Path SumHard
• CFChoosing Capital for Treelandrerooting with edge directions1700
• CFMaximum White Subtreecorethe standard rerooting problem1800
• CFDistance in Treedp[v][d] = vertices at depth d below v1800
• LCSum of Distances in Treethe second program of this lessonHard
• CFTree with Maximum Coststretch; same rerooting formula, different weights1900

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCFair Distribution of Cookiesn is tiny; brute force is possible, bitmask DP is the clean versionMedium
• LCMinimum Number of Work Sessions to Finish the Tasksdp[mask] = best (sessions, load)Medium
• LCPartition to K Equal Sum SubsetsMedium
• CFKefa and Dishescoredp[mask][last]1800
• CSESElevator Ridescoredp[mask] stores a pair, rides and the load of the last ride
• CSESHamiltonian Flightscorecount paths that visit every city once
• LCShortest Path Visiting All NodesBFS over (mask, node) statesHard
• CFLooking for Orderfix the lowest unset bit to avoid permutations2000
• CFRoman and Numbersmask of used digits plus remainder2000
• CFLittle Pony and Harmony Cheststretch; mask of used primes2000

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESCounting Numberscorethe program of this lesson
• LCNumbers At Most N Given Digit SetHard
• LCNon-negative Integers without Consecutive OnesHard
• LCCount Special Integerscorestate = mask of used digitsHard
• LCNumbers With Repeated Digitscount the complementHard
• CFClassy Numberscoreat most 3 non-zero digits, count in a huge range1900
• LCNumber of Digit Onecount how often a digit appears, not just how many numbersHard
• CFMagic Numbersposition parity and remainder modulo m2200
• CFSegment Sumstretch; at most k distinct digits, and you need the sum, not the count2300
• CFBeautiful numbersstretch; remainder modulo lcm(1..9) = 25202500

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESStatic Range Minimum Queriesthe exact template of this lesson
• CFRorororobotrange maximum over columns decides if a path is blocked1700
• CFIntegers Have Friendscoregcd of adjacent differences, two pointers1800
• CFMax GEQ Sumcoresparse table for max, monotonic stack for the borders1800
• CFArray Stabilization (GCD version)coreduplicate the array, binary search on the number of steps1900
• CFCGCDSSQcount subarrays by gcd value2000
• CFFriends and Subsequencesbinary search where max - min is checked in O(1)2100

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCRange Sum Query - MutableMedium
• CSESList Removalsfind the k-th remaining element, then delete it
• CFPashmak and Parmida's problemcorecount pairs with prefix count greater than suffix count1800
• CFEnemy is weakcorefor each middle element count smaller on the left, greater on the right1900
• CFMultisetcoreinsert, delete the k-th smallest; descend the tree1900
• LCCount of Smaller Numbers After SelfHard
• LCReverse Pairscompress values and count with a treeHard
• CFInfinite Inversionscompress the touched positions, count the gaps between them2100
• CFYaroslav and Divisorsoffline queries sorted by right end2200

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESDynamic Range Sum Queriesthe exact template of this lesson
• CSESDynamic Range Minimum Queries
• CFDistinct Characters Queriescorestore a 26-bit mask per node and merge with OR1600
• CFXenia and Bit Operationscorethe merge operation alternates by level1700
• CFInteresting Arrayone tree per bit, or AND-merge with a build step1800
• CFSereja and Bracketscorenode = (matched pairs, open left, close right)2000
• CFThe Child and Sequencekeep max per node, skip segments where max < mod2300

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESRange Update Queriesrange add, point query
• CFCopying Datacorerange assign, point query; the latest copy wins1900
• CFXOR on Segmentcoreone tree per bit, lazy flip2000
• CSESRange Updates and Sumsadd and assign tags together
• LCMy Calendar IIIrange add, global maximumHard
• CFCircular RMQcorerange add, range min, wrapped ranges2200
• LCFalling Squaresrange chmax-assign, range maxHard
• CFLucky Queriesnode keeps counts of 4 and 7; lazy flip and swap2400

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESDistinct Values Queriescorethe textbook Mo's problem
• CFLittle Elephant and Arraycorecount values x that appear exactly x times in a range1800
• CFXOR and Favorite NumbercoreMo with prefix XORs2200
• CFPowerful arraymaintain a sum of cnt^2 * value2200
• CFAnn and Books2300
• CFTree and QueriesMo on the Euler tour order2400
• CFOne Occurrencesolvable with Mo, intended with sweep and segment tree2400

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCLowest Common Ancestor of a Binary Treethe plain recursive version, to see the definition at workMedium
• CSESCompany Queries IILCA of two employees
• CSESCompany Queries Ik-th ancestor
• CSESDistance Queries
• LCKth Ancestor of a Tree NodeHard
• CFTree Queriescorea path from the root through the deepest node must pass within one step of every query node1900
• CFFools and RoadscoreLCA plus a difference array on the tree1900
• CF1-Trees and Queriescoreshortest walk with one extra edge is the best of three distances2000
• CFA and B and Lecture Roomsthe middle of a path, then subtree sizes2100
• CFMinimum spanning tree for each edgestore the maximum edge on each jump2100

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESSubtree Queriesthe exact template of this lesson
• CSESPath Queriesroot-to-node sums, range add and point query
• CFPropagating treecoreflip the sign of an update by depth parity2000
• CFDanil and a Part-time Jobcoreflip a whole subtree with a lazy segment tree2000
• CFBlood Cousinslists of tin per depth, then binary search2100
• CFNew Year Treecoreassign a colour to a subtree, count distinct colours with a bitmask2100
• CFLomsat gelraldominant colour of every subtree; the flat array enables the sack technique2300

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFPath Queriesprocess edges by weight, merge components, track the pair count1800
• LCAccounts Mergemerge lists of emails, smaller into largerMedium
• CSESDistinct Colorscorenumber of distinct colours in every subtree
• CFTree Requestscoreletter counts per depth, can the letters form a palindrome2200
• CFDominant Indicescoremost frequent depth in each subtree2300
• CFBlood Cousins Return2400

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCFind the Index of the First Occurrence in a Stringrolling hash as a warm-upEasy
• CFPassworda prefix that is also a suffix and appears in the middle1700
• CFGood Substringscorecount distinct substrings by their hashes1800
• CFPrefix-Suffix Palindrome (Hard version)hashes of the string and its reverse give palindrome checks1800
• CFWatto and Mechanismcorefor each query, try changing each position and look up the hash2000
• CFCompress Wordslongest overlap of a suffix and a prefix2000
• LCLongest Duplicate Substringcorebinary search on the length, a set of window hashesHard
• LCDistinct Echo SubstringsHard

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CSESString Matchingcount the occurrences of a pattern
• CSESFinding Bordersall borders of a string
• CSESFinding Periods
• CFMUH and Cube Wallscorematch differences of neighbouring heights, not the heights1800
• LCLongest Happy Prefixthe last value of the prefix functionHard
• CFTavas and Malekascorecheck overlaps with the Z-function, count free positions1900
• CFPrefixes and Suffixescoreall borders, each counted by how often it occurs2000
• LCSum of Scores of Built Stringsthe sum of the Z-functionHard
• CFAnthem of Berlandprefix function as an automaton inside a DP2300

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCImplement Trie (Prefix Tree)coreMedium
• LCMaximum XOR of Two Numbers in an ArraycoreMedium
• CFVasiliy's Multisetcoreadd, remove and max-XOR queries; keep a count on every node1800
• CFA Lot of Gameswin/lose states on the nodes of a trie1900
• CFVitya and Strange LessonXOR every element by x, then find the mex with a trie2000
• CFChoosing The Commandercount numbers whose XOR with p is below l2000
• LCWord Search IIHard
• CFBeautiful Subarrayscoreprefix XORs plus counting pairs with XOR at least k2100
• CFSausage Maximization2200

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCNim GameEasy
• CSESStick Gamewin/lose DP over positions
• CSESNim Game I
• CFNumber Gamecorereason about the last move before reaching for theory1400
• CFDeleting Divisorscoreprint win/lose for small n and find the pattern1700
• CFCircle Game1700
• CFStoned Game1800
• LCCan I Winwin/lose states over a bitmaskMedium
• CFGame of StonescoreGrundy value of each pile, then XOR2100
• CFLieges of LegendreGrundy values with a repeating pattern2200

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCKnight Probability in Chessboardprobability DP over stepsMedium
• CFJourneycoreexpected path length on a tree, one DFS1500
• CFRandom Eventsthe answer is 1 minus a product of probabilities1500
• CFLittle Pony and Expected MaximumcoreE[max] = sum over k of P(max >= k)1600
• CFIlya and Escalatorexpected value DP over time1700
• CFBag of mice1800
• LCNew 21 Gameprobability DP with a sliding window sumMedium
• CFLet's Play Osu!coremaintain the expected value of a running streak2000
• CFWish I Knew How to Sortcoreexpected-value DP with a self-loop, answer mod 9982443532000
• CFGame on Treelinearity, one indicator per vertex2200

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFJzzhu and Sequencesthe recurrence has a short period, but try the matrix as practice1300
• CSESFibonacci Numbers
• CFTetrahedroncorecount walks of length n on four vertices1500
• CSESGraph Paths Icore
• CFAdd Oneusually done with DP; matrix power also works1600
• LCKnight Dialerwalks on a 10-vertex graphMedium
• CFXor-sequencescorebuild the adjacency matrix from a condition on pairs1900
• CFDecoding Genome1900
• CFWet Shark and Blocks2000
• CFMagic Gemscoref(n) = f(n-1) + f(n-m), a state of size m2100
• CFSasha and Arraymatrices stored in a segment tree2300

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• CFStrongly Connected Citywarm-up, every vertex must reach every other1400
• CSESFlight Routes Check
• CSESPlanets and Kingdomsprint the SCC id of every vertex
• CFCheckpostscorecheapest vertex in each SCC, count the ways1700
• CFReachability from the Capitalcorecount source components in the condensation2000
• CFBertown roadscoreorient every edge along the DFS; a bridge makes it impossible2000
• CSESCoin Collectorcondense, then DP on the DAG
• LCCritical Connections in a NetworkcoreHard
• CFWe Need More Bossescontract the 2-edge-connected components, take the tree diameter2100
• CFTourist Reform2300

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCGuess Number Higher or Lowerthe same idea behind an API callEasy
• CFLost Numbersfour queries, then brute force the order1400
• CFBear and Prime 1001400
• CFRuler (easy version)binary search on a monotone reply1500
• CFGuess the K-th Zero (Easy version)corethe same idea as the example below1600
• CFGuessing the Greatest (easy version)corefind the second maximum with a range query1600
• CFChocolate Bunny1600
• CFFixed Point Guessing1600
• CFSearching Local Minimumcorebinary search where the "direction" is a comparison of neighbours1700
• CFRuler (hard version)1700
• CFGuessing the Greatest (hard version)1900

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDesign TwitterMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDesign a Food Rating SystemMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDesign Browser HistoryMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCPrint in OrderEasy
• LCPrint FooBar AlternatelyMedium
• LCThe Dining PhilosophersMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDesign Underground SystemMedium
• LCDesign Authentication ManagerMedium
• LCTime Based Key-Value StoreMedium
• LCDesign SpreadsheetMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDesign Parking SystemEasy

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCLFU CacheHard
• LCLRU CacheMedium

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCDesign Movie Rental SystemHard

Easiest first. Give each one an honest 30–45 minutes before reading a hint or the editorial. Core problems are the minimum.

• LCStock Price Fluctuation Medium