Hash Table LeetCode Problems: All 588,
With Python Solutions

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

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Easy: 143
Medium: 347
Hard: 98

1 pattern

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.

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python

Please analyze this context and help me answer the question or solve the problem.

Technical interview · 45 minsolution.pyShared editor · Python 3Longest substring without repeatsMediumGiven a string s, return the length of the longest contiguous substring in which every character is unique.s = "abcabcbb"
→ 31def length_of_longest_substring(s: str) -> int:2 last_seen = {}3 left = best = 04 for right, ch in enumerate(s):5 if ch in last_seen and last_seen[ch] >= left:

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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python

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python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

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python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

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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)

Easy problems
No.	Problem	Topics
0001	Two Sum	Array, Hash Table
0013	Roman to Integer	Hash Table, Math, String
0141	Linked List Cycle	Hash Table, Linked List, Two Pointers
0160	Intersection of Two Linked Lists	Hash Table, Linked List, Two Pointers
0169	Majority Element	Array, Hash Table, Divide and Conquer +2
0202	Happy Number	Hash Table, Math, Two Pointers
0205	Isomorphic Strings	Hash Table, String
0217	Contains Duplicate	Array, Hash Table, Sorting
0219	Contains Duplicate II	Array, Hash Table, Sliding Window
0242	Valid Anagram	Hash Table, String, Sorting
0268	Missing Number	Bit Manipulation, Array, Hash Table +3
0290	Word Pattern	Hash Table, String
0349	Intersection of Two Arrays	Array, Hash Table, Two Pointers +2
0350	Intersection of Two Arrays II	Array, Hash Table, Two Pointers +2
0383	Ransom Note	Hash Table, String, Counting
0387	First Unique Character in a String	Queue, Hash Table, String +1
0389	Find the Difference	Bit Manipulation, Hash Table, String +1
0409	Longest Palindrome	Greedy, Hash Table, String
0448	Find All Numbers Disappeared in an Array	Array, Hash Table
0496	Next Greater Element I	Stack, Array, Hash Table +1
0500	Keyboard Row	Array, Hash Table, String
0575	Distribute Candies	Array, Hash Table
0594	Longest Harmonious Subsequence	Array, Hash Table, Counting +2
0599	Minimum Index Sum of Two Lists	Array, Hash Table, String
0645	Set Mismatch	Bit Manipulation, Array, Hash Table +1
0653	Two Sum IV - Input is a BST	Tree, Depth-First Search, Breadth-First Search +4
0697	Degree of an Array	Array, Hash Table
0705	Design HashSet	Design, Array, Hash Table +2
0706	Design HashMap	Design, Array, Hash Table +2
0748	Shortest Completing Word	Array, Hash Table, String
0771	Jewels and Stones	Hash Table, String
0804	Unique Morse Code Words	Array, Hash Table, String
0819	Most Common Word	Array, Hash Table, String +1
0859	Buddy Strings	Hash Table, String
0884	Uncommon Words from Two Sentences	Hash Table, String, Counting
0888	Fair Candy Swap	Array, Hash Table, Binary Search +1
0914	X of a Kind in a Deck of Cards	Array, Hash Table, Math +2
0929	Unique Email Addresses	Array, Hash Table, String
0953	Verifying an Alien Dictionary	Array, Hash Table, String
0961	N-Repeated Element in Size 2N Array	Array, Hash Table
0997	Find the Town Judge	Graph, Array, Hash Table
1002	Find Common Characters	Array, Hash Table, String
1122	Relative Sort Array	Array, Hash Table, Counting Sort +1
1128	Number of Equivalent Domino Pairs	Array, Hash Table, Counting
1160	Find Words That Can Be Formed by Characters	Array, Hash Table, String +1
1189	Maximum Number of Balloons	Hash Table, String, Counting
1207	Unique Number of Occurrences	Array, Hash Table

Medium (115)

Medium problems
No.	Problem	Topics
0003	Longest Substring Without Repeating Characters	Hash Table, String, Sliding Window
0012	Integer to Roman	Hash Table, Math, String
0017	Letter Combinations of a Phone Number	Hash Table, String, Backtracking
0036	Valid Sudoku	Array, Hash Table, Matrix
0049	Group Anagrams	Array, Hash Table, String +1
0073	Set Matrix Zeroes	Array, Hash Table, Matrix
0105	Construct Binary Tree from Preorder and Inorder Traversal	Tree, Array, Hash Table +2
0106	Construct Binary Tree from Inorder and Postorder Traversal	Tree, Array, Hash Table +2
0128	Longest Consecutive Sequence	Union Find, Array, Hash Table
0133	Clone Graph	Depth-First Search, Breadth-First Search, Graph +1
0138	Copy List with Random Pointer	Hash Table, Linked List
0139	Word Break	Trie, Memoization, Array +3
0142	Linked List Cycle II	Hash Table, Linked List, Two Pointers
0146	LRU Cache	Design, Hash Table, Linked List +1
0166	Fraction to Recurring Decimal	Hash Table, Math, String
0187	Repeated DNA Sequences	Bit Manipulation, Hash Table, String +3
0208	Implement Trie (Prefix Tree)	Design, Trie, Hash Table +1
0229	Majority Element II	Array, Hash Table, Counting +1
0264	Ugly Number II	Hash Table, Math, Dynamic Programming +1
0299	Bulls and Cows	Hash Table, String, Counting
0347	Top K Frequent Elements	Array, Hash Table, Divide and Conquer +5
0355	Design Twitter	Design, Hash Table, Linked List +1
0380	Insert Delete GetRandom O(1)	Design, Array, Hash Table +2
0395	Longest Substring with At Least K Repeating Characters	Hash Table, String, Divide and Conquer +1
0398	Random Pick Index	Reservoir Sampling, Hash Table, Math +1
0421	Maximum XOR of Two Numbers in an Array	Bit Manipulation, Trie, Array +1
0423	Reconstruct Original Digits from English	Hash Table, Math, String
0424	Longest Repeating Character Replacement	Hash Table, String, Sliding Window
0433	Minimum Genetic Mutation	Breadth-First Search, Hash Table, String
0438	Find All Anagrams in a String	Hash Table, String, Sliding Window
0442	Find All Duplicates in an Array	Array, Hash Table, Sorting
0447	Number of Boomerangs	Array, Hash Table, Math
0451	Sort Characters By Frequency	Hash Table, String, Bucket Sort +3
0454	4Sum II	Array, Hash Table
0457	Circular Array Loop	Array, Hash Table, Two Pointers
0491	Non-decreasing Subsequences	Bit Manipulation, Array, Hash Table +1
0508	Most Frequent Subtree Sum	Tree, Depth-First Search, Hash Table +1
0519	Random Flip Matrix	Reservoir Sampling, Hash Table, Math +1
0522	Longest Uncommon Subsequence II	Array, Hash Table, Two Pointers +2
0523	Continuous Subarray Sum	Array, Hash Table, Math +1
0525	Contiguous Array	Array, Hash Table, Prefix Sum
0532	K-diff Pairs in an Array	Array, Hash Table, Two Pointers +2
0535	Encode and Decode TinyURL	Design, Hash Table, String +1
0554	Brick Wall	Array, Hash Table
0560	Subarray Sum Equals K	Array, Hash Table, Prefix Sum
0567	Permutation in String	Hash Table, Two Pointers, String +1
0609	Find Duplicate File in System	Array, Hash Table, String
0621	Task Scheduler	Greedy, Array, Hash Table +3
0648	Replace Words	Trie, Array, Hash Table +1
0652	Find Duplicate Subtrees	Tree, Depth-First Search, Hash Table +1
0659	Split Array into Consecutive Subsequences	Greedy, Array, Hash Table +1
0676	Implement Magic Dictionary	Depth-First Search, Design, Trie +2
0677	Map Sum Pairs	Design, Trie, Hash Table +1
0690	Employee Importance	Tree, Depth-First Search, Breadth-First Search +2
0692	Top K Frequent Words	Trie, Array, Hash Table +5
0720	Longest Word in Dictionary	Trie, Array, Hash Table +2
0721	Accounts Merge	Depth-First Search, Breadth-First Search, Union Find +4
0740	Delete and Earn	Array, Hash Table, Dynamic Programming
0752	Open the Lock	Breadth-First Search, Array, Hash Table +1
0756	Pyramid Transition Matrix	Bit Manipulation, Hash Table, String +1
0763	Partition Labels	Greedy, Hash Table, Two Pointers +1
0767	Reorganize String	Greedy, Hash Table, String +3
0781	Rabbits in Forest	Greedy, Array, Hash Table +1
0791	Custom Sort String	Hash Table, String, Sorting
0792	Number of Matching Subsequences	Trie, Array, Hash Table +4
0811	Subdomain Visit Count	Array, Hash Table, String +1
0817	Linked List Components	Array, Hash Table, Linked List
0820	Short Encoding of Words	Trie, Array, Hash Table +1
0822	Card Flipping Game	Array, Hash Table
0823	Binary Trees With Factors	Array, Hash Table, Dynamic Programming +1
0833	Find And Replace in String	Array, Hash Table, String +1
0840	Magic Squares In Grid	Array, Hash Table, Math +1
0846	Hand of Straights	Greedy, Array, Hash Table +1
0863	All Nodes Distance K in Binary Tree	Tree, Depth-First Search, Breadth-First Search +2
0865	Smallest Subtree with all the Deepest Nodes	Tree, Depth-First Search, Breadth-First Search +2
0869	Reordered Power of 2	Hash Table, Math, Counting +2
0873	Length of Longest Fibonacci Subsequence	Array, Hash Table, Dynamic Programming
0874	Walking Robot Simulation	Array, Hash Table, Simulation
0889	Construct Binary Tree from Preorder and Postorder Traversal	Tree, Array, Hash Table +2
0890	Find and Replace Pattern	Array, Hash Table, String
0893	Groups of Special-Equivalent Strings	Array, Hash Table, String +1
0904	Fruit Into Baskets	Array, Hash Table, Sliding Window
0911	Online Election	Design, Array, Hash Table +1
0916	Word Subsets	Array, Hash Table, String
0923	3Sum With Multiplicity	Array, Hash Table, Two Pointers +2
0930	Binary Subarrays With Sum	Array, Hash Table, Prefix Sum +1
0939	Minimum Area Rectangle	Geometry, Array, Hash Table +2
0947	Most Stones Removed with Same Row or Column	Depth-First Search, Union Find, Graph +1
0954	Array of Doubled Pairs	Greedy, Array, Hash Table +1
0959	Regions Cut By Slashes	Depth-First Search, Breadth-First Search, Union Find +3
0963	Minimum Area Rectangle II	Geometry, Array, Hash Table +1
0966	Vowel Spellchecker	Array, Hash Table, String
0970	Powerful Integers	Hash Table, Math, Enumeration
0974	Subarray Sums Divisible by K	Array, Hash Table, Prefix Sum
0981	Time Based Key-Value Store	Design, Hash Table, String +1
1010	Pairs of Songs With Total Durations Divisible by 60	Array, Hash Table, Counting
1015	Smallest Integer Divisible by K	Hash Table, Math
1016	Binary String With Substrings Representing 1 To N	Bit Manipulation, Hash Table, String +1
1027	Longest Arithmetic Subsequence	Array, Hash Table, Binary Search +1
1048	Longest String Chain	Array, Hash Table, Two Pointers +3
1054	Distant Barcodes	Greedy, Array, Hash Table +3
1072	Flip Columns For Maximum Number of Equal Rows	Array, Hash Table, Matrix
1079	Letter Tile Possibilities	Hash Table, String, Backtracking +1
1090	Largest Values From Labels	Greedy, Array, Hash Table +2
1110	Delete Nodes And Return Forest	Tree, Depth-First Search, Array +2
1123	Lowest Common Ancestor of Deepest Leaves	Tree, Depth-First Search, Breadth-First Search +2
1124	Longest Well-Performing Interval	Stack, Array, Hash Table +2
1138	Alphabet Board Path	Hash Table, String
1146	Snapshot Array	Design, Array, Hash Table +1
1156	Swap For Longest Repeated Character Substring	Hash Table, String, Sliding Window
1169	Invalid Transactions	Array, Hash Table, String +1
1170	Compare Strings by Frequency of the Smallest Character	Array, Hash Table, String +2
1171	Remove Zero Sum Consecutive Nodes from Linked List	Hash Table, Linked List
1177	Can Make Palindrome from Substring	Bit Manipulation, Array, Hash Table +2
1202	Smallest String With Swaps	Depth-First Search, Breadth-First Search, Union Find +4

Hard (38)

Hard problems
No.	Problem	Topics
0030	Substring with Concatenation of All Words	Hash Table, String, Sliding Window
0037	Sudoku Solver	Array, Hash Table, Backtracking +1
0041	First Missing Positive	Array, Hash Table
0076	Minimum Window Substring	Hash Table, String, Sliding Window
0126	Word Ladder II	Breadth-First Search, Hash Table, String +1
0127	Word Ladder	Breadth-First Search, Hash Table, String
0140	Word Break II	Trie, Memoization, Array +4
0149	Max Points on a Line	Geometry, Array, Hash Table +1
0336	Palindrome Pairs	Trie, Array, Hash Table +1
0352	Data Stream as Disjoint Intervals	Union Find, Design, Hash Table +3
0381	Insert Delete GetRandom O(1) - Duplicates allowed	Design, Array, Hash Table +2
0391	Perfect Rectangle	Geometry, Array, Hash Table +2
0432	All O`one Data Structure	Design, Hash Table, Linked List +1
0460	LFU Cache	Design, Hash Table, Linked List +1
0480	Sliding Window Median	Array, Hash Table, Sliding Window +1
0632	Smallest Range Covering Elements from K Lists	Greedy, Array, Hash Table +3
0691	Stickers to Spell Word	Bit Manipulation, Memoization, Array +5
0710	Random Pick with Blacklist	Array, Hash Table, Math +3
0736	Parse Lisp Expression	Stack, Recursion, Hash Table +1
0745	Prefix and Suffix Search	Design, Trie, Array +2
0815	Bus Routes	Breadth-First Search, Array, Hash Table
0828	Count Unique Characters of All Substrings of a Given String	Hash Table, String, Dynamic Programming
0839	Similar String Groups	Depth-First Search, Breadth-First Search, Union Find +3
0854	K-Similar Strings	Breadth-First Search, Hash Table, String
0895	Maximum Frequency Stack	Stack, Design, Hash Table +1
0924	Minimize Malware Spread	Depth-First Search, Breadth-First Search, Union Find +3
0928	Minimize Malware Spread II	Depth-First Search, Breadth-First Search, Union Find +3
0952	Largest Component Size by Common Factor	Union Find, Array, Hash Table +2
0982	Triples with Bitwise AND Equal To Zero	Bit Manipulation, Array, Hash Table
0987	Vertical Order Traversal of a Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
0992	Subarrays with K Different Integers	Array, Hash Table, Counting +1
0996	Number of Squareful Arrays	Bit Manipulation, Array, Hash Table +4
1001	Grid Illumination	Array, Hash Table
1036	Escape a Large Maze	Depth-First Search, Breadth-First Search, Array +1
1074	Number of Submatrices That Sum to Target	Array, Hash Table, Matrix +1
1096	Brace Expansion II	Stack, Breadth-First Search, Hash Table +3
1172	Dinner Plate Stacks	Stack, Design, Hash Table +1
1178	Number of Valid Words for Each Puzzle	Bit Manipulation, Trie, Array +2

How Hash Table problems are solved

When the Hash Table problem arrives live.

Approach

Approach

Approach

Hash Table problems by difficulty

Easy (47)47

Medium (115)115

Hard (38)38

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Easy (47)

Medium (115)

Hard (38)