Hash Table LeetCode Problems: All 588, With Python Solutions

Every problem in this library that LeetCode tags Hash Table — 588 in total, 492 of them with a complete Python solution, a worked example and the time and space complexity of the approach.

588 problems
143 Easy
347 Medium
98 Hard

How Hash Table problems are solved

A tag names the subject, not the method. These pattern hubs cover the techniques that actually solve Hash Table problems — each one explains the approach, gives a Python template and states its complexity.

Hash Map — Trade memory for time: remember what you have seen so the second pass never happens.

Hash Table problems by difficulty

Showing the first 200 of 588 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (47)

#	Problem	Difficulty	Topics
1	Two Sum	Easy	Array, Hash Table
13	Roman to Integer	Easy	Hash Table, Math, String
141	Linked List Cycle	Easy	Hash Table, Linked List, Two Pointers
160	Intersection of Two Linked Lists	Easy	Hash Table, Linked List, Two Pointers
169	Majority Element	Easy	Array, Hash Table, Divide and Conquer +2
202	Happy Number	Easy	Hash Table, Math, Two Pointers
205	Isomorphic Strings	Easy	Hash Table, String
217	Contains Duplicate	Easy	Array, Hash Table, Sorting
219	Contains Duplicate II	Easy	Array, Hash Table, Sliding Window
242	Valid Anagram	Easy	Hash Table, String, Sorting
268	Missing Number	Easy	Bit Manipulation, Array, Hash Table +3
290	Word Pattern	Easy	Hash Table, String
349	Intersection of Two Arrays	Easy	Array, Hash Table, Two Pointers +2
350	Intersection of Two Arrays II	Easy	Array, Hash Table, Two Pointers +2
383	Ransom Note	Easy	Hash Table, String, Counting
387	First Unique Character in a String	Easy	Queue, Hash Table, String +1
389	Find the Difference	Easy	Bit Manipulation, Hash Table, String +1
409	Longest Palindrome	Easy	Greedy, Hash Table, String
448	Find All Numbers Disappeared in an Array	Easy	Array, Hash Table
496	Next Greater Element I	Easy	Stack, Array, Hash Table +1
500	Keyboard Row	Easy	Array, Hash Table, String
575	Distribute Candies	Easy	Array, Hash Table
594	Longest Harmonious Subsequence	Easy	Array, Hash Table, Counting +2
599	Minimum Index Sum of Two Lists	Easy	Array, Hash Table, String
645	Set Mismatch	Easy	Bit Manipulation, Array, Hash Table +1
653	Two Sum IV - Input is a BST	Easy	Tree, Depth-First Search, Breadth-First Search +4
697	Degree of an Array	Easy	Array, Hash Table
705	Design HashSet	Easy	Design, Array, Hash Table +2
706	Design HashMap	Easy	Design, Array, Hash Table +2
748	Shortest Completing Word	Easy	Array, Hash Table, String
771	Jewels and Stones	Easy	Hash Table, String
804	Unique Morse Code Words	Easy	Array, Hash Table, String
819	Most Common Word	Easy	Array, Hash Table, String +1
859	Buddy Strings	Easy	Hash Table, String
884	Uncommon Words from Two Sentences	Easy	Hash Table, String, Counting
888	Fair Candy Swap	Easy	Array, Hash Table, Binary Search +1
914	X of a Kind in a Deck of Cards	Easy	Array, Hash Table, Math +2
929	Unique Email Addresses	Easy	Array, Hash Table, String
953	Verifying an Alien Dictionary	Easy	Array, Hash Table, String
961	N-Repeated Element in Size 2N Array	Easy	Array, Hash Table
997	Find the Town Judge	Easy	Graph, Array, Hash Table
1002	Find Common Characters	Easy	Array, Hash Table, String
1122	Relative Sort Array	Easy	Array, Hash Table, Counting Sort +1
1128	Number of Equivalent Domino Pairs	Easy	Array, Hash Table, Counting
1160	Find Words That Can Be Formed by Characters	Easy	Array, Hash Table, String +1
1189	Maximum Number of Balloons	Easy	Hash Table, String, Counting
1207	Unique Number of Occurrences	Easy	Array, Hash Table

Medium (115)

#	Problem	Difficulty	Topics
3	Longest Substring Without Repeating Characters	Medium	Hash Table, String, Sliding Window
12	Integer to Roman	Medium	Hash Table, Math, String
17	Letter Combinations of a Phone Number	Medium	Hash Table, String, Backtracking
36	Valid Sudoku	Medium	Array, Hash Table, Matrix
49	Group Anagrams	Medium	Array, Hash Table, String +1
73	Set Matrix Zeroes	Medium	Array, Hash Table, Matrix
105	Construct Binary Tree from Preorder and Inorder Traversal	Medium	Tree, Array, Hash Table +2
106	Construct Binary Tree from Inorder and Postorder Traversal	Medium	Tree, Array, Hash Table +2
128	Longest Consecutive Sequence	Medium	Union Find, Array, Hash Table
133	Clone Graph	Medium	Depth-First Search, Breadth-First Search, Graph +1
138	Copy List with Random Pointer	Medium	Hash Table, Linked List
139	Word Break	Medium	Trie, Memoization, Array +3
142	Linked List Cycle II	Medium	Hash Table, Linked List, Two Pointers
146	LRU Cache	Medium	Design, Hash Table, Linked List +1
166	Fraction to Recurring Decimal	Medium	Hash Table, Math, String
187	Repeated DNA Sequences	Medium	Bit Manipulation, Hash Table, String +3
208	Implement Trie (Prefix Tree)	Medium	Design, Trie, Hash Table +1
229	Majority Element II	Medium	Array, Hash Table, Counting +1
264	Ugly Number II	Medium	Hash Table, Math, Dynamic Programming +1
299	Bulls and Cows	Medium	Hash Table, String, Counting
347	Top K Frequent Elements	Medium	Array, Hash Table, Divide and Conquer +5
355	Design Twitter	Medium	Design, Hash Table, Linked List +1
380	Insert Delete GetRandom O(1)	Medium	Design, Array, Hash Table +2
395	Longest Substring with At Least K Repeating Characters	Medium	Hash Table, String, Divide and Conquer +1
398	Random Pick Index	Medium	Reservoir Sampling, Hash Table, Math +1
421	Maximum XOR of Two Numbers in an Array	Medium	Bit Manipulation, Trie, Array +1
423	Reconstruct Original Digits from English	Medium	Hash Table, Math, String
424	Longest Repeating Character Replacement	Medium	Hash Table, String, Sliding Window
433	Minimum Genetic Mutation	Medium	Breadth-First Search, Hash Table, String
438	Find All Anagrams in a String	Medium	Hash Table, String, Sliding Window
442	Find All Duplicates in an Array	Medium	Array, Hash Table, Sorting
447	Number of Boomerangs	Medium	Array, Hash Table, Math
451	Sort Characters By Frequency	Medium	Hash Table, String, Bucket Sort +3
454	4Sum II	Medium	Array, Hash Table
457	Circular Array Loop	Medium	Array, Hash Table, Two Pointers
491	Non-decreasing Subsequences	Medium	Bit Manipulation, Array, Hash Table +1
508	Most Frequent Subtree Sum	Medium	Tree, Depth-First Search, Hash Table +1
519	Random Flip Matrix	Medium	Reservoir Sampling, Hash Table, Math +1
522	Longest Uncommon Subsequence II	Medium	Array, Hash Table, Two Pointers +2
523	Continuous Subarray Sum	Medium	Array, Hash Table, Math +1
525	Contiguous Array	Medium	Array, Hash Table, Prefix Sum
532	K-diff Pairs in an Array	Medium	Array, Hash Table, Two Pointers +2
535	Encode and Decode TinyURL	Medium	Design, Hash Table, String +1
554	Brick Wall	Medium	Array, Hash Table
560	Subarray Sum Equals K	Medium	Array, Hash Table, Prefix Sum
567	Permutation in String	Medium	Hash Table, Two Pointers, String +1
609	Find Duplicate File in System	Medium	Array, Hash Table, String
621	Task Scheduler	Medium	Greedy, Array, Hash Table +3
648	Replace Words	Medium	Trie, Array, Hash Table +1
652	Find Duplicate Subtrees	Medium	Tree, Depth-First Search, Hash Table +1
659	Split Array into Consecutive Subsequences	Medium	Greedy, Array, Hash Table +1
676	Implement Magic Dictionary	Medium	Depth-First Search, Design, Trie +2
677	Map Sum Pairs	Medium	Design, Trie, Hash Table +1
690	Employee Importance	Medium	Tree, Depth-First Search, Breadth-First Search +2
692	Top K Frequent Words	Medium	Trie, Array, Hash Table +5
720	Longest Word in Dictionary	Medium	Trie, Array, Hash Table +2
721	Accounts Merge	Medium	Depth-First Search, Breadth-First Search, Union Find +4
740	Delete and Earn	Medium	Array, Hash Table, Dynamic Programming
752	Open the Lock	Medium	Breadth-First Search, Array, Hash Table +1
756	Pyramid Transition Matrix	Medium	Bit Manipulation, Hash Table, String +1
763	Partition Labels	Medium	Greedy, Hash Table, Two Pointers +1
767	Reorganize String	Medium	Greedy, Hash Table, String +3
781	Rabbits in Forest	Medium	Greedy, Array, Hash Table +1
791	Custom Sort String	Medium	Hash Table, String, Sorting
792	Number of Matching Subsequences	Medium	Trie, Array, Hash Table +4
811	Subdomain Visit Count	Medium	Array, Hash Table, String +1
817	Linked List Components	Medium	Array, Hash Table, Linked List
820	Short Encoding of Words	Medium	Trie, Array, Hash Table +1
822	Card Flipping Game	Medium	Array, Hash Table
823	Binary Trees With Factors	Medium	Array, Hash Table, Dynamic Programming +1
833	Find And Replace in String	Medium	Array, Hash Table, String +1
840	Magic Squares In Grid	Medium	Array, Hash Table, Math +1
846	Hand of Straights	Medium	Greedy, Array, Hash Table +1
863	All Nodes Distance K in Binary Tree	Medium	Tree, Depth-First Search, Breadth-First Search +2
865	Smallest Subtree with all the Deepest Nodes	Medium	Tree, Depth-First Search, Breadth-First Search +2
869	Reordered Power of 2	Medium	Hash Table, Math, Counting +2
873	Length of Longest Fibonacci Subsequence	Medium	Array, Hash Table, Dynamic Programming
874	Walking Robot Simulation	Medium	Array, Hash Table, Simulation
889	Construct Binary Tree from Preorder and Postorder Traversal	Medium	Tree, Array, Hash Table +2
890	Find and Replace Pattern	Medium	Array, Hash Table, String
893	Groups of Special-Equivalent Strings	Medium	Array, Hash Table, String +1
904	Fruit Into Baskets	Medium	Array, Hash Table, Sliding Window
911	Online Election	Medium	Design, Array, Hash Table +1
916	Word Subsets	Medium	Array, Hash Table, String
923	3Sum With Multiplicity	Medium	Array, Hash Table, Two Pointers +2
930	Binary Subarrays With Sum	Medium	Array, Hash Table, Prefix Sum +1
939	Minimum Area Rectangle	Medium	Geometry, Array, Hash Table +2
947	Most Stones Removed with Same Row or Column	Medium	Depth-First Search, Union Find, Graph +1
954	Array of Doubled Pairs	Medium	Greedy, Array, Hash Table +1
959	Regions Cut By Slashes	Medium	Depth-First Search, Breadth-First Search, Union Find +3
963	Minimum Area Rectangle II	Medium	Geometry, Array, Hash Table +1
966	Vowel Spellchecker	Medium	Array, Hash Table, String
970	Powerful Integers	Medium	Hash Table, Math, Enumeration
974	Subarray Sums Divisible by K	Medium	Array, Hash Table, Prefix Sum
981	Time Based Key-Value Store	Medium	Design, Hash Table, String +1
1010	Pairs of Songs With Total Durations Divisible by 60	Medium	Array, Hash Table, Counting
1015	Smallest Integer Divisible by K	Medium	Hash Table, Math
1016	Binary String With Substrings Representing 1 To N	Medium	Bit Manipulation, Hash Table, String +1
1027	Longest Arithmetic Subsequence	Medium	Array, Hash Table, Binary Search +1
1048	Longest String Chain	Medium	Array, Hash Table, Two Pointers +3
1054	Distant Barcodes	Medium	Greedy, Array, Hash Table +3
1072	Flip Columns For Maximum Number of Equal Rows	Medium	Array, Hash Table, Matrix
1079	Letter Tile Possibilities	Medium	Hash Table, String, Backtracking +1
1090	Largest Values From Labels	Medium	Greedy, Array, Hash Table +2
1110	Delete Nodes And Return Forest	Medium	Tree, Depth-First Search, Array +2
1123	Lowest Common Ancestor of Deepest Leaves	Medium	Tree, Depth-First Search, Breadth-First Search +2
1124	Longest Well-Performing Interval	Medium	Stack, Array, Hash Table +2
1138	Alphabet Board Path	Medium	Hash Table, String
1146	Snapshot Array	Medium	Design, Array, Hash Table +1
1156	Swap For Longest Repeated Character Substring	Medium	Hash Table, String, Sliding Window
1169	Invalid Transactions	Medium	Array, Hash Table, String +1
1170	Compare Strings by Frequency of the Smallest Character	Medium	Array, Hash Table, String +2
1171	Remove Zero Sum Consecutive Nodes from Linked List	Medium	Hash Table, Linked List
1177	Can Make Palindrome from Substring	Medium	Bit Manipulation, Array, Hash Table +2
1202	Smallest String With Swaps	Medium	Depth-First Search, Breadth-First Search, Union Find +4

Hard (38)

#	Problem	Difficulty	Topics
30	Substring with Concatenation of All Words	Hard	Hash Table, String, Sliding Window
37	Sudoku Solver	Hard	Array, Hash Table, Backtracking +1
41	First Missing Positive	Hard	Array, Hash Table
76	Minimum Window Substring	Hard	Hash Table, String, Sliding Window
126	Word Ladder II	Hard	Breadth-First Search, Hash Table, String +1
127	Word Ladder	Hard	Breadth-First Search, Hash Table, String
140	Word Break II	Hard	Trie, Memoization, Array +4
149	Max Points on a Line	Hard	Geometry, Array, Hash Table +1
336	Palindrome Pairs	Hard	Trie, Array, Hash Table +1
352	Data Stream as Disjoint Intervals	Hard	Union Find, Design, Hash Table +3
381	Insert Delete GetRandom O(1) - Duplicates allowed	Hard	Design, Array, Hash Table +2
391	Perfect Rectangle	Hard	Geometry, Array, Hash Table +2
432	All O`one Data Structure	Hard	Design, Hash Table, Linked List +1
460	LFU Cache	Hard	Design, Hash Table, Linked List +1
480	Sliding Window Median	Hard	Array, Hash Table, Sliding Window +1
632	Smallest Range Covering Elements from K Lists	Hard	Greedy, Array, Hash Table +3
691	Stickers to Spell Word	Hard	Bit Manipulation, Memoization, Array +5
710	Random Pick with Blacklist	Hard	Array, Hash Table, Math +3
736	Parse Lisp Expression	Hard	Stack, Recursion, Hash Table +1
745	Prefix and Suffix Search	Hard	Design, Trie, Array +2
815	Bus Routes	Hard	Breadth-First Search, Array, Hash Table
828	Count Unique Characters of All Substrings of a Given String	Hard	Hash Table, String, Dynamic Programming
839	Similar String Groups	Hard	Depth-First Search, Breadth-First Search, Union Find +3
854	K-Similar Strings	Hard	Breadth-First Search, Hash Table, String
895	Maximum Frequency Stack	Hard	Stack, Design, Hash Table +1
924	Minimize Malware Spread	Hard	Depth-First Search, Breadth-First Search, Union Find +3
928	Minimize Malware Spread II	Hard	Depth-First Search, Breadth-First Search, Union Find +3
952	Largest Component Size by Common Factor	Hard	Union Find, Array, Hash Table +2
982	Triples with Bitwise AND Equal To Zero	Hard	Bit Manipulation, Array, Hash Table
987	Vertical Order Traversal of a Binary Tree	Hard	Tree, Depth-First Search, Breadth-First Search +3
992	Subarrays with K Different Integers	Hard	Array, Hash Table, Counting +1
996	Number of Squareful Arrays	Hard	Bit Manipulation, Array, Hash Table +4
1001	Grid Illumination	Hard	Array, Hash Table
1036	Escape a Large Maze	Hard	Depth-First Search, Breadth-First Search, Array +1
1074	Number of Submatrices That Sum to Target	Hard	Array, Hash Table, Matrix +1
1096	Brace Expansion II	Hard	Stack, Breadth-First Search, Hash Table +3
1172	Dinner Plate Stacks	Hard	Stack, Design, Hash Table +1
1178	Number of Valid Words for Each Puzzle	Hard	Bit Manipulation, Trie, Array +2

When the Hash Table problem arrives live

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