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C001 Find a target pairArrays & hashing Easy ↗ C002 Repeated readingArrays & hashing Easy ↗ C003 One profitable tradeArrays & hashing Easy ↗ C004 Clean palindromeStrings Easy ↗ C005 Same letter inventoryStrings Easy ↗ C006 First unique symbolStrings Easy ↗ C007 Merge two sorted runsTwo pointers Easy ↗ C008 Distinct sorted valuesTwo pointers Easy ↗ C009 Locate a sorted keyBinary search Easy ↗ C010 Insertion boundaryBinary search Easy ↗ C011 Missing sequence memberBit manipulation Easy ↗ C012 Move empty readingsTwo pointers Easy ↗ C013 Reverse word orderStrings Easy ↗ C014 Shared prefixStrings Easy ↗ C015 Balanced bracketsStacks Easy ↗ C016 Running stack minimumStacks Easy ↗ C017 Range totalsPrefix sums Easy ↗ C018 Steps to the landingDynamic programming Easy ↗ C019 Unpaired valueBit manipulation Easy ↗ C020 Majority readingArrays & hashing Easy ↗ C021 Integer square rootMath Easy ↗ C022 Increment digit arrayArrays & hashing Easy ↗ C023 Best contiguous totalDynamic programming Easy ↗ C024 Can one room hold them?Intervals Easy ↗ C025 Transpose the gridMatrices Easy ↗ C026 Paint a connected regionGraphs Easy ↗ C027 Height of a binary treeTrees Easy ↗ C028 Mirror equalityTrees Easy ↗ C029 Cycle in next pointersLinked lists Easy ↗ C030 Count set bitsBit manipulation Easy ↗ C031 Triples summing to zeroArrays & hashing Medium ↗ C032 Longest distinct windowSliding windows Medium ↗ C033 Group matching inventoriesArrays & hashing Medium ↗ C034 Products without the current entryPrefix sums Medium ↗ C035 Count target-sum segmentsPrefix sums Medium ↗ C036 Unite overlapping intervalsIntervals Medium 🔒 C037 Search a rotated runBinary search Medium 🔒 C038 Most frequent valuesHeaps Medium 🔒 C039 Fewest coinsDynamic programming Medium 🔒 C040 Nonadjacent rewardsDynamic programming Medium 🔒 C041 Count islandsGraphs Medium 🔒 C042 Can all courses finish?Graphs Medium 🔒 C043 Read a tree by levelTrees Medium 🔒 C044 Strict search-tree validationTrees Medium 🔒 C045 Wait for a warmer dayStacks Medium 🔒 C046 Increasing subsequence lengthDynamic programming Medium 🔒 C047 Segment a messageDynamic programming Medium 🔒 C048 Generate subsetsBacktracking Medium 🔒 C049 Reusable sum combinationsBacktracking Medium 🔒 C050 Shortest positive-sum windowSliding windows Medium 🔒 C051 Largest water containerTwo pointers Medium 🔒 C052 Read a spiralMatrices Medium 🔒 C053 Zero marked rows and columnsMatrices Medium 🔒 C054 Minimum meeting roomsHeaps Medium 🔒 C055 Least-recently-used cacheDesign Medium 🔒 C056 Prefix dictionaryDesign Medium 🔒 C057 Signal arrival timeGraphs Medium 🔒 C058 Minimum wiring costGraphs Medium 🔒 C059 Insert a reservation intervalIntervals Medium 🔒 C060 Longest palindromic substringStrings Medium 🔒 C061 Decode nested repetitionStacks Medium 🔒 C062 Reach the last positionGreedy Medium 🔒 C063 Grid paths with obstaclesDynamic programming Medium 🔒 C064 Reverse a pointer chainLinked lists Medium 🔒 C065 Lowest shared ancestorTrees Medium 🔒 C066 Maximum of every windowSliding windows Hard ↗ C067 Smallest covering substringSliding windows Hard ↗ C068 Median across two sorted arraysBinary search Hard ↗ C069 Rain between barsTwo pointers Hard 🔒 C070 Largest histogram rectangleStacks Hard 🔒 C071 Edit one string into anotherDynamic programming Hard 🔒 C072 Full match with dot and starDynamic programming Hard 🔒 C073 Wildcard filename matchingDynamic programming Hard 🔒 C074 Shortest word transformationGraphs Hard 🔒 C075 Merge many sorted feedsHeaps Hard 🔒 C076 Round-trip tree encodingTrees Hard 🔒 C077 Maximum sum through a treeTrees Hard 🔒 C078 Repeated ancestor queriesTrees Hard 🔒 C079 Live range sumsRange queries Hard 🔒 C080 Live interval minimumRange queries Hard 🔒 C081 Count out-of-order pairsDivide & conquer Hard 🔒 C082 Maximum-value scheduleDynamic programming Hard 🔒 C083 Burst for maximum rewardDynamic programming Hard 🔒 C084 Minimum palindrome cutsDynamic programming Hard 🔒 C085 Count safe queen placementsBacktracking Hard 🔒 C086 Find dictionary words on a boardBacktracking Hard 🔒 C087 Recover an alien alphabetGraphs Hard 🔒 C088 Critical network edgesGraphs Hard 🔒 C089 Strongly connected groupsGraphs Hard 🔒 C090 Maximum network throughputGraphs Hard 🔒 C091 Shortest tour through all stopsBitmask DP Hard 🔒 C092 Assign workers to tasksBitmask DP Hard 🔒 C093 Count the digit oneDigit counting Hard 🔒 C094 Smaller values to the rightRange queries Hard 🔒 C095 Islands appearing over timeDisjoint sets Hard 🔒 C096 Median of every moving windowRange queries Hard 🔒 C097 Largest all-one rectangleMatrices Hard 🔒 C098 Shortest qualifying sum with negativesSliding windows Hard 🔒 C099 Median after each arrivalHeaps Hard 🔒 C100 At most k completed tradesDynamic programming Hard 🔒 About this collection & research sources Original practice questions covering arrays, strings, trees, graphs, dynamic programming, and data structures. Each question includes constraints, examples, a worked explanation, complexity analysis, and a Python 3 reference implementation. These are independent practice problems, not a reproduction of the LeetCode question bank.
Difficulty is an editorial guide. These are practice problems, not claims about questions asked by a particular firm. Read the stated assumptions carefully; a different model can change the answer.