Hash Table
149 problems · 109 with full explanations
20 Easy78 Medium11 Hard
Hash tables provide O(1) average-case lookup, insert, and delete. They are the go-to tool for counting frequencies, detecting complements (Two Sum pattern), and caching seen values. In C#, use Dictionary<K,V> for maps and HashSet<T> for membership checks.
How to practice
To practice Hash Table problems effectively, start with the Easy problems listed below, trace through each solution on paper, then re-implement without looking. When you can recognise the hash table pattern within 30 seconds of reading a new problem, move on to Medium difficulty. Use the related topic pages and our study guide for a structured progression.
Start here (Easy + explained)
All Hash Table problems
- 30.Substring with Concatenation of All WordsHard
- 36.Valid SudokuMedium
- 73.Set Matrix ZeroesMedium
- 76.Minimum Window SubstringHard
- 106.Construct Binary Tree from Inorder and Postorder TraversalMedium
- 126.Word Ladder IIHard
- 146.LRU CacheMedium
- 160.Intersection of Two Linked ListsEasy
- 166.Fraction to Recurring DecimalMedium
- 171.Excel Sheet Column NumberEasy
- 202.Happy NumberEasy
- 219.Contains Duplicate IIEasy
- 264.Ugly Number IIMedium
- 299.Bulls and CowsUnknown
- 350.Intersection of Two Arrays IIEasy
- 432.All O`one Data StructureUnknown
- 460.LFU CacheHard
- 500.Keyboard RowEasy
- 508.Most Frequent Subtree SumMedium
- 554.Brick WallMedium
- 567.Permutation in StringMedium
- 594.Longest Harmonious SubsequenceEasy
- 632.Smallest Range Covering Elements from K ListsHard
- 706.Design HashMapUnknown
- 726.Number of AtomsHard
- 752.Open the LockMedium
- 756.Pyramid Transition MatrixUnknown
- 763.Partition LabelsMedium
- 773.Sliding PuzzleHard
- 820.Short Encoding of WordsMedium
- 821.Shortest Distance to a CharacterEasy
- 822.Card Flipping GameUnknown
- 827.Making A Large IslandHard
- 840.Magic Squares In GridMedium
- 862.Shortest Subarray with Sum at Least KHard
- 869.Reordered Power of 2Medium
- 873.Length of Longest Fibonacci SubsequenceMedium
- 874.Walking Robot SimulationMedium
- 884.Uncommon Words from Two SentencesEasy
- 889.Construct Binary Tree from Preorder and Postorder TraversalMedium
- 890.Find and Replace PatternMedium
- 904.Fruit Into BasketsMedium
- 909.Snakes and LaddersMedium
- 916.Word SubsetsMedium
- 961.N-Repeated Element in Size 2N ArrayUnknown
- 966.Vowel SpellcheckerMedium
- 981.Time Based Key-Value StoreMedium
- 1015.Smallest Integer Divisible by KMedium
- 1061.Lexicographically Smallest Equivalent StringMedium
- 1072.Flip Columns For Maximum Number of Equal RowsMedium
- 1079.Letter Tile PossibilitiesMedium
- 1123.Lowest Common Ancestor of Deepest LeavesMedium
- 1128.Number of Equivalent Domino PairsEasy
- 1248.Count Number of Nice SubarraysMedium
- 1261.Find Elements in a Contaminated Binary TreeMedium
- 1331.Rank Transform of an ArrayUnknown
- 1346.Check If N and Its Double ExistUnknown
- 1358.Number of Substrings Containing All Three CharactersMedium
- 1371.Find the Longest Substring Containing Vowels in Even CountsMedium
- 1380.Lucky Numbers in a MatrixEasy
- 1394.Find Lucky Integer in an ArrayUnknown
- 1399.Count Largest GroupUnknown
- 1400.Construct K Palindrome StringsUnknown
- 1410.HTML Entity ParserUnknown
- 1461.Check If a String Contains All Binary Codes of Size KMedium
- 1487.Making File Names UniqueUnknown
- 1488.Avoid Flood in The CityUnknown
- 1497.Check If Array Pairs Are Divisible by kMedium
- 1590.Make Sum Divisible by PMedium
- 1593.Split a String Into the Max Number of Unique SubstringsMedium
- 1636.Sort Array by Increasing FrequencyUnknown
- 1684.Count the Number of Consistent StringsEasy
- 1695.Maximum Erasure ValueMedium
- 1726.Tuple with Same ProductMedium
- 1790.Check if One String Swap Can Make Strings EqualEasy
- 1857.Largest Color Value in a Directed GraphHard
- 1865.Finding Pairs With a Certain SumUnknown
- 1912.Design Movie Rental SystemUnknown
- 1930.Unique Length-3 Palindromic SubsequencesMedium
- 1935.Maximum Number of Words You Can TypeUnknown
- 1942.The Number of the Smallest Unoccupied ChairUnknown
- 1948.Delete Duplicate Folders in SystemUnknown
- 1980.Find Unique Binary StringMedium
- 1995.Count Special QuadrupletsUnknown
- 2007.Find Original Array From Doubled ArrayUnknown
- 2043.Simple Bank SystemUnknown
- 2053.Kth Distinct String in an ArrayUnknown
- 2094.Finding 3-Digit Even NumbersUnknown
- 2115.Find All Possible Recipes from Given SuppliesUnknown
- 2131.Longest Palindrome by Concatenating Two Letter WordsMedium
- 2154.Keep Multiplying Found Values by TwoEasy
- 2176.Count Equal and Divisible Pairs in an ArrayUnknown
- 2191.Sort the Jumbled NumbersUnknown
- 2196.Create Binary Tree From DescriptionsMedium
- 2206.Divide Array Into Equal PairsEasy
- 2225.Find Players With Zero or One LossesMedium
- 2273.Find Resultant Array After Removing AnagramsUnknown
- 2275.Largest Combination With Bitwise AND Greater Than ZeroMedium
- 2342.Max Sum of a Pair With Equal Sum of DigitsMedium
- 2349.Design a Number Container SystemMedium
- 2353.Design a Food Rating SystemMedium
- 2364.Count Number of Bad PairsHard
- 2370.Longest Ideal SubsequenceMedium
- 2418.Sort the PeopleEasy
- 2425.Bitwise XOR of All PairingsMedium
- 2461.Maximum Sum of Distinct Subarrays With Length KMedium
- 2491.Divide Players Into Teams of Equal SkillMedium
- 2501.Longest Square Streak in an ArrayMedium
- 2516.Take K of Each Character From Left and RightMedium
- 2537.Count the Number of Good SubarraysMedium
- 2554.Maximum Number of Integers to Choose From a Range IMedium
- 2570.Merge Two 2D Arrays by Summing ValuesMedium
- 2598.Smallest Missing Non-negative Integer After OperationsUnknown
- 2641.Cousins in Binary Tree IIMedium
- 2657.Find the Prefix Common Array of Two ArraysMedium
- 2661.First Completely Painted Row or ColumnMedium
- 2707.Extra Characters in a StringMedium
- 2718.Sum of Matrix After QueriesUnknown
- 2762.Continuous SubarraysMedium
- 2780.Minimum Index of a Valid SplitMedium
- 2799.Count Complete Subarrays in an ArrayMedium
- 2839.Check if Strings Can be Made Equal With Operations IEasy
- 2840.Check if Strings Can be Made Equal With Operations IIMedium
- 2845.Count of Interesting SubarraysMedium
- 2965.Find Missing and Repeated ValuesUnknown
- 2981.Find Longest Special Substring That Occurs Thrice IMedium
- 3005.Count Elements With Maximum FrequencyEasy
- 3016.Minimum Number of Pushes to Type Word IIMedium
- 3085.Minimum Deletions to Make String K-SpecialEasy
- 3160.Find the Number of Distinct Colors Among the BallsMedium
- 3186.Maximum Total Damage With Spell CastingMedium
- 3217.Delete Nodes From Linked List Present in ArrayMedium
- 3223.Minimum Length of String After OperationsUnknown
- 3272.Find the Count of Good IntegersUnknown
- 3289.The Two Sneaky Numbers of DigitvilleMedium
- 3306.Count of Substrings Containing Every Vowel and K Consonants IIMedium
- 3318.Find X-Sum of All K-Long Subarrays IEasy
- 3321.Find X-Sum of All K-Long Subarrays IIUnknown
- 3335.Total Characters in String After Transformations IMedium
- 3337.Total Characters in String After Transformations IIUnknown
- 3375.Minimum Operations to Make Array Values Equal to KMedium
- 3396.Minimum Number of Operations to Make Elements in Array DistinctMedium
- 3408.Design Task ManagerUnknown
- 3442.Maximum Difference Between Even and Odd Frequency IMedium
- 3443.Maximum Manhattan Distance After K ChangesUnknown
- 3445.Maximum Difference Between Even and Odd Frequency IIMedium
- 3484.Design SpreadsheetMedium
- 3508.Implement RouterUnknown
- 3541.Find Most Frequent Vowel and ConsonantUnknown