Pattern 01 of 22

Hash Map Pattern: Template + 709 LeetCode Problems

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

Easy: 227
Medium: 372
Hard: 110

What the hash map pattern is

A hash map converts a search into a lookup, which is why it is the most common single answer in this entire corpus — 588 of the 3000 problems are built on one. Three uses cover nearly all of them. The complement lookup asks, at each element, whether the thing that would complete the answer has already been seen: storing values as you go turns two nested loops into one pass, and the reason it works is that a pair is found from its second member, so only the past needs to be remembered. Frequency counting answers anagram, majority and top-k questions by comparing counts instead of arrangements. Grouping needs a canonical key — a sorted string, a character-count tuple, a normalised pattern — chosen so that two inputs collide exactly when they belong together, and that choice is the whole problem. Python's dict preserves insertion order, so an ordered result often needs no extra bookkeeping.

When to use it

The brute force is a nested loop asking "is there another element such that…".
You are counting occurrences, or comparing two multisets.
Items must be grouped by a property you can compute into a stable key.
Membership must be tested repeatedly and the input is not sorted.
Seen-before state has to survive across a single pass — de-duplication, cycle detection on values.

The hash map template in Python

The shape, not a solution to any one problem. Adapt the condition and the summary being maintained; the skeleton stays the same across the 709 problems listed below.

PythonHash Map template

def two_sum(nums, target):
    seen = {}                            # value -> the index it was found at

    for i, value in enumerate(nums):
        if target - value in seen:       # the partner was already scanned
            return [seen[target - value], i]
        seen[value] = i

    return []

Complexity characteristics

Time: O(1) average per operation, O(n) worst case
Auxiliary space: O(n)

Average-case constant lookups are what turn a nested-loop search into a single pass, so the complement and frequency-counting solutions are O(n) time and O(n) space. The worst case is linear per operation when every key collides, which is irrelevant on real data and is the reason a language's string hash is randomised per process. Ordering is not free: getting a sorted result out of a map costs the sort on top.

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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.

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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.

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All 709 hash map LeetCode problems

Every problem in the library the hash map pattern applies to, grouped by LeetCode's own difficulty rating. 601 of the 709 carry a complete Python solution with a worked example and complexity analysis; the rest are listed for completeness, with the LeetCode Premium ones marked.

Easy (227)

Easy problems
No.	Problem	Topics
0001	Two Sum	Array, Hash Table
0013	Roman to Integer	Hash Table, Math, String
0058	Length of Last Word	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
0434	Number of Segments in a String	String
0448	Find All Numbers Disappeared in an Array	Array, Hash Table
0459	Repeated Substring Pattern	String, String Matching
0482	License Key Formatting	String
0496	Next Greater Element I	Stack, Array, Hash Table +1
0500	Keyboard Row	Array, Hash Table, String
0520	Detect Capital	String
0521	Longest Uncommon Subsequence I	String
0551	Student Attendance Record I	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
0657	Robot Return to Origin	String, Simulation
0697	Degree of an Array	Array, Hash Table
0705	Design HashSet	Design, Array, Hash Table +2
0706	Design HashMap	Design, Array, Hash Table +2
0709	To Lower Case	String
0748	Shortest Completing Word	Array, Hash Table, String
1207	Unique Number of Occurrences	Array, Hash Table
2215	Find the Difference of Two Arrays	Array, Hash Table
0170	Two Sum III - Data structure designPremium	Design, Array, Hash Table +2
0243	Shortest Word DistancePremium	Array, String
0246	Strobogrammatic NumberPremium	Hash Table, Two Pointers, String
0266	Palindrome PermutationPremium	Bit Manipulation, Hash Table, String
0293	Flip GamePremium	String
0359	Logger Rate LimiterPremium	Design, Hash Table, Data Stream
0604	Design Compressed String IteratorPremium	Design, Array, String +1
0734	Sentence SimilarityPremium	Array, Hash Table, String
0760	Find Anagram MappingsPremium	Array, Hash Table
0771	Jewels and Stones	Hash Table, String
0796	Rotate String	String, String Matching
0804	Unique Morse Code Words	Array, Hash Table, String
0806	Number of Lines To Write String	Array, String
0819	Most Common Word	Array, Hash Table, String +1
0824	Goat Latin	String
0830	Positions of Large Groups	String
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
0944	Delete Columns to Make Sorted	Array, 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
1078	Occurrences After Bigram	String
1086	High FivePremium	Array, Hash Table, Sorting +1
1108	Defanging an IP Address	String
1119	Remove Vowels from a StringPremium	String
1122	Relative Sort Array	Array, Hash Table, Counting Sort +1
1128	Number of Equivalent Domino Pairs	Array, Hash Table, Counting
1133	Largest Unique NumberPremium	Array, Hash Table, Sorting
1160	Find Words That Can Be Formed by Characters	Array, Hash Table, String +1
1165	Single-Row KeyboardPremium	Hash Table, String
1189	Maximum Number of Balloons	Hash Table, String, Counting
1213	Intersection of Three Sorted ArraysPremium	Array, Hash Table, Binary Search +1
1221	Split a String in Balanced Strings	Greedy, String, Counting
1275	Find Winner on a Tic Tac Toe Game	Array, Hash Table, Matrix +1
1309	Decrypt String from Alphabet to Integer Mapping	String
1331	Rank Transform of an Array	Array, Hash Table, Sorting
1346	Check If N and Its Double Exist	Array, Hash Table, Two Pointers +2
1356	Sort Integers by The Number of 1 Bits	Bit Manipulation, Array, Counting +1
1365	How Many Numbers Are Smaller Than the Current Number	Array, Hash Table, Counting Sort +1
1370	Increasing Decreasing String	Hash Table, String, Counting
1374	Generate a String With Characters That Have Odd Counts	String
1394	Find Lucky Integer in an Array	Array, Hash Table, Counting
1399	Count Largest Group	Hash Table, Math
1408	String Matching in an Array	Array, String, String Matching
1417	Reformat The String	String
1426	Counting ElementsPremium	Array, Hash Table
1436	Destination City	Array, Hash Table, String
1446	Consecutive Characters	String
1460	Make Two Arrays Equal by Reversing Subarrays	Array, Hash Table, Sorting
1496	Path Crossing	Hash Table, String
1507	Reformat Date	String
1512	Number of Good Pairs	Array, Hash Table, Math +1
1528	Shuffle String	Array, String
1556	Thousand Separator	String
1576	Replace All ?'s to Avoid Consecutive Repeating Characters	String
1592	Rearrange Spaces Between Words	String
1603	Design Parking System	Design, Counting, Simulation
1624	Largest Substring Between Two Equal Characters	Hash Table, String
1629	Slowest Key	Array, String
1636	Sort Array by Increasing Frequency	Array, Hash Table, Sorting
1640	Check Array Formation Through Concatenation	Array, Hash Table
1656	Design an Ordered Stream	Design, Array, Hash Table +1
1662	Check If Two String Arrays are Equivalent	Array, String
1678	Goal Parser Interpretation	String
1684	Count the Number of Consistent Strings	Bit Manipulation, Array, Hash Table +2
1694	Reformat Phone Number	String
1704	Determine if String Halves Are Alike	String, Counting
1742	Maximum Number of Balls in a Box	Hash Table, Math, Counting
1748	Sum of Unique Elements	Array, Hash Table, Counting
1758	Minimum Changes To Make Alternating Binary String	String
1763	Longest Nice Substring	Bit Manipulation, Hash Table, String +2
1773	Count Items Matching a Rule	Array, String
1784	Check if Binary String Has at Most One Segment of Ones	String
1790	Check if One String Swap Can Make Strings Equal	Hash Table, String, Counting
1796	Second Largest Digit in a String	Hash Table, String
1805	Number of Different Integers in a String	Hash Table, String
1816	Truncate Sentence	Array, String
1832	Check if the Sentence Is Pangram	Hash Table, String
1844	Replace All Digits with Characters	String
1854	Maximum Population Year	Array, Counting, Prefix Sum
1869	Longer Contiguous Segments of Ones than Zeros	String
1876	Substrings of Size Three with Distinct Characters	Hash Table, String, Counting +1
1880	Check if Word Equals Summation of Two Words	String
1893	Check if All the Integers in a Range Are Covered	Array, Hash Table, Prefix Sum
1897	Redistribute Characters to Make All Strings Equal	Hash Table, String, Counting
1933	Check if String Is Decomposable Into Value-Equal SubstringsPremium	String
1935	Maximum Number of Words You Can Type	Hash Table, String
1941	Check if All Characters Have Equal Number of Occurrences	Hash Table, String, Counting
1945	Sum of Digits of String After Convert	String, Simulation
1957	Delete Characters to Make Fancy String	String
1967	Number of Strings That Appear as Substrings in Word	Array, String
1995	Count Special Quadruplets	Array, Hash Table, Enumeration
2006	Count Number of Pairs With Absolute Difference K	Array, Hash Table, Counting
2011	Final Value of Variable After Performing Operations	Array, String, Simulation
2032	Two Out of Three	Bit Manipulation, Array, Hash Table
2042	Check if Numbers Are Ascending in a Sentence	String
2047	Number of Valid Words in a Sentence	String
2053	Kth Distinct String in an Array	Array, Hash Table, String +1
2062	Count Vowel Substrings of a String	Hash Table, String
2068	Check Whether Two Strings are Almost Equivalent	Hash Table, String, Counting
2085	Count Common Words With One Occurrence	Array, Hash Table, String +1
2094	Finding 3-Digit Even Numbers	Recursion, Array, Hash Table +2
2099	Find Subsequence of Length K With the Largest Sum	Array, Hash Table, Sorting +1
2103	Rings and Rods	Hash Table, String
2114	Maximum Number of Words Found in Sentences	Array, String
2124	Check if All A's Appears Before All B's	String
2129	Capitalize the Title	String
2133	Check if Every Row and Column Contains All Numbers	Array, Hash Table, Matrix
2138	Divide a String Into Groups of Size k	String, Simulation
2148	Count Elements With Strictly Smaller and Greater Elements	Array, Counting, Sorting
2154	Keep Multiplying Found Values by Two	Array, Hash Table, Sorting +1
2185	Counting Words With a Given Prefix	Array, String, String Matching
2190	Most Frequent Number Following Key In an Array	Array, Hash Table, Counting
2194	Cells in a Range on an Excel Sheet	String
2206	Divide Array Into Equal Pairs	Bit Manipulation, Array, Hash Table +1
2229	Check if an Array Is ConsecutivePremium	Array, Hash Table, Sorting
2243	Calculate Digit Sum of a String	String, Simulation
2248	Intersection of Multiple Arrays	Array, Hash Table, Counting +1
2255	Count Prefixes of a Given String	Array, String
2264	Largest 3-Same-Digit Number in String	String
2273	Find Resultant Array After Removing Anagrams	Array, Hash Table, String +1
2278	Percentage of Letter in String	String
2283	Check if Number Has Equal Digit Count and Digit Value	Hash Table, String, Counting
2287	Rearrange Characters to Make Target String	Hash Table, String, Counting
2299	Strong Password Checker II	String
2309	Greatest English Letter in Upper and Lower Case	Hash Table, String, Enumeration
2315	Count Asterisks	String
2325	Decode the Message	Hash Table, String
2341	Maximum Number of Pairs in Array	Array, Hash Table, Counting
2347	Best Poker Hand	Array, Hash Table, Counting
2351	First Letter to Appear Twice	Bit Manipulation, Hash Table, String +1
2357	Make Array Zero by Subtracting Equal Amounts	Greedy, Array, Hash Table +3
2363	Merge Similar Items	Array, Hash Table, Ordered Set +1
2367	Number of Arithmetic Triplets	Array, Hash Table, Two Pointers +1
2395	Find Subarrays With Equal Sum	Array, Hash Table
2399	Check Distances Between Same Letters	Array, Hash Table, String
2404	Most Frequent Even Element	Array, Hash Table, Counting
2418	Sort the People	Array, Hash Table, String +1
2423	Remove Letter To Equalize Frequency	Hash Table, String, Counting
2437	Number of Valid Clock Times	String, Enumeration
2441	Largest Positive Integer That Exists With Its Negative	Array, Hash Table, Two Pointers +1
2446	Determine if Two Events Have Conflict	Array, String
2451	Odd String Difference	Array, Hash Table, String
2465	Number of Distinct Averages	Array, Hash Table, Two Pointers +1
2475	Number of Unequal Triplets in Array	Array, Hash Table, Sorting
2490	Circular Sentence	String
2496	Maximum Value of a String in an Array	Array, String
2506	Count Pairs Of Similar Strings	Bit Manipulation, Array, Hash Table +2
2515	Shortest Distance to Target String in a Circular Array	Array, String
2529	Maximum Count of Positive Integer and Negative Integer	Array, Binary Search, Counting
2540	Minimum Common Value	Array, Hash Table, Two Pointers +1
2549	Count Distinct Numbers on Board	Array, Hash Table, Math +1
2570	Merge Two 2D Arrays by Summing Values	Array, Hash Table, Two Pointers
2586	Count the Number of Vowel Strings in Range	Array, String, Counting
2605	Form Smallest Number From Two Digit Arrays	Array, Hash Table, Enumeration
2609	Find the Longest Balanced Substring of a Binary String	String
2670	Find the Distinct Difference Array	Array, Hash Table
2678	Number of Senior Citizens	Array, String
2682	Find the Losers of the Circular Game	Array, Hash Table, Simulation
2710	Remove Trailing Zeros From a String	String
2716	Minimize String Length	Hash Table, String
2729	Check if The Number is Fascinating	Hash Table, Math
2744	Find Maximum Number of String Pairs	Array, Hash Table, String +1
2748	Number of Beautiful Pairs	Array, Hash Table, Math +2
2784	Check if Array is Good	Array, Hash Table, Sorting
2788	Split Strings by Separator	Array, String
2810	Faulty Keyboard	String, Simulation
2815	Max Pair Sum in an Array	Array, Hash Table
2828	Check if a String Is an Acronym of Words	Array, String
2833	Furthest Point From Origin	String, Counting
2839	Check if Strings Can be Made Equal With Operations I	String
2848	Points That Intersect With Cars	Array, Hash Table, Prefix Sum
2869	Minimum Operations to Collect Elements	Bit Manipulation, Array, Hash Table
2913	Subarrays Distinct Element Sum of Squares I	Array, Hash Table
2932	Maximum Strong Pair XOR I	Bit Manipulation, Trie, Array +2
2937	Make Three Strings Equal	String
2942	Find Words Containing Character	Array, String
2956	Find Common Elements Between Two Arrays	Array, Hash Table
2960	Count Tested Devices After Test Operations	Array, Counting, Simulation
2965	Find Missing and Repeated Values	Array, Hash Table, Math +1
2996	Smallest Missing Integer Greater Than Sequential Prefix Sum	Array, Hash Table, Sorting

Medium (372)

Medium problems
No.	Problem	Topics
0003	Longest Substring Without Repeating Characters	Hash Table, String, Sliding Window
0006	Zigzag Conversion	String
0008	String to Integer (atoi)	String
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
0038	Count and Say	String
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
0468	Validate IP Address	String
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
0686	Repeated String Match	String, String Matching
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
0722	Remove Comments	Array, String
0740	Delete and Earn	Array, Hash Table, Dynamic Programming
0763	Partition Labels	Greedy, Hash Table, Two Pointers +1
0846	Hand of Straights	Greedy, Array, Hash Table +1
0981	Time Based Key-Value Store	Design, Hash Table, String +1
1657	Determine if Two Strings Are Close	Hash Table, String, Counting +1
1679	Max Number of K-Sum Pairs	Array, Hash Table, Two Pointers +1
2013	Detect Squares	Design, Array, Hash Table +1
2336	Smallest Number in Infinite Set	Design, Hash Table, Ordered Set +1
2352	Equal Row and Column Pairs	Array, Hash Table, Matrix +1
0159	Longest Substring with At Most Two Distinct CharactersPremium	Hash Table, String, Sliding Window
0244	Shortest Word Distance IIPremium	Design, Array, Hash Table +2
0245	Shortest Word Distance IIIPremium	Array, String
0247	Strobogrammatic Number IIPremium	Recursion, Array, String
0249	Group Shifted StringsPremium	Array, Hash Table, String
0267	Palindrome Permutation IIPremium	Hash Table, String, Backtracking
0271	Encode and Decode StringsPremium	Design, Array, String
0288	Unique Word AbbreviationPremium	Design, Array, Hash Table +1
0291	Word Pattern IIPremium	Hash Table, String, Backtracking
0311	Sparse Matrix MultiplicationPremium	Array, Hash Table, Matrix
0314	Binary Tree Vertical Order TraversalPremium	Tree, Depth-First Search, Breadth-First Search +3
0325	Maximum Size Subarray Sum Equals kPremium	Array, Hash Table, Prefix Sum
0340	Longest Substring with At Most K Distinct CharactersPremium	Hash Table, String, Sliding Window
0348	Design Tic-Tac-ToePremium	Design, Array, Hash Table +2
0353	Design Snake GamePremium	Design, Queue, Array +2
0356	Line ReflectionPremium	Array, Hash Table, Math
0379	Design Phone DirectoryPremium	Design, Queue, Array +2
0531	Lonely Pixel IPremium	Array, Hash Table, Matrix
0533	Lonely Pixel IIPremium	Array, Hash Table, Matrix
0544	Output Contest MatchesPremium	Recursion, String, Simulation
0582	Kill ProcessPremium	Tree, Depth-First Search, Breadth-First Search +2
0616	Add Bold Tag in StringPremium	Trie, Array, Hash Table +2
0635	Design Log Storage SystemPremium	Design, Hash Table, String +1
0666	Path Sum IVPremium	Tree, Depth-First Search, Array +2
0681	Next Closest TimePremium	Hash Table, String, Backtracking +1
0694	Number of Distinct IslandsPremium	Depth-First Search, Breadth-First Search, Union Find +2
0737	Sentence Similarity IIPremium	Depth-First Search, Breadth-First Search, Union Find +3
0752	Open the Lock	Breadth-First Search, Array, Hash Table +1
0756	Pyramid Transition Matrix	Bit Manipulation, Hash Table, String +1
0758	Bold Words in StringPremium	Trie, Array, Hash Table +2
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
0831	Masking Personal Information	String
0833	Find And Replace in String	Array, Hash Table, String +1
0840	Magic Squares In Grid	Array, Hash Table, Math +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
0900	RLE Iterator	Design, Array, Counting +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
0945	Minimum Increment to Make Array Unique	Greedy, Array, Counting +1
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
0957	Prison Cells After N Days	Bit Manipulation, 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
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
1100	Find K-Length Substrings With No Repeated CharactersPremium	Hash Table, String, Sliding Window
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
1152	Analyze User Website Visit PatternPremium	Array, Hash Table, String +1
1156	Swap For Longest Repeated Character Substring	Hash Table, String, Sliding Window
1166	Design File SystemPremium	Design, Trie, Hash Table +1
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
1181	Before and After PuzzlePremium	Array, Hash Table, String +1
1198	Find Smallest Common Element in All RowsPremium	Array, Hash Table, Binary Search +2
1202	Smallest String With Swaps	Depth-First Search, Breadth-First Search, Union Find +4
1218	Longest Arithmetic Subsequence of Given Difference	Array, Hash Table, Dynamic Programming
1244	Design A LeaderboardPremium	Design, Hash Table, Sorting
1248	Count Number of Nice Subarrays	Array, Hash Table, Math +2
1257	Smallest Common RegionPremium	Tree, Depth-First Search, Breadth-First Search +3
1258	Synonymous SentencesPremium	Sort, Union Find, Array +3
1261	Find Elements in a Contaminated Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
1267	Count Servers that Communicate	Depth-First Search, Breadth-First Search, Union Find +3
1282	Group the People Given the Group Size They Belong To	Greedy, Array, Hash Table
1296	Divide Array in Sets of K Consecutive Numbers	Greedy, Array, Hash Table +1
1297	Maximum Number of Occurrences of a Substring	Hash Table, String, Sliding Window
1311	Get Watched Videos by Your Friends	Breadth-First Search, Graph, Array +2
1324	Print Words Vertically	Array, String, Simulation
1338	Reduce Array Size to The Half	Greedy, Array, Hash Table +2
1347	Minimum Number of Steps to Make Two Strings Anagram	Hash Table, String, Counting
1348	Tweet Counts Per Frequency	Design, Hash Table, String +3
1357	Apply Discount Every n Orders	Design, Array, Hash Table
1358	Number of Substrings Containing All Three Characters	Hash Table, String, Sliding Window
1366	Rank Teams by Votes	Array, Hash Table, String +2
1371	Find the Longest Substring Containing Vowels in Even Counts	Bit Manipulation, Hash Table, String +1
1386	Cinema Seat Allocation	Greedy, Bit Manipulation, Array +1
1396	Design Underground System	Design, Hash Table, String
1400	Construct K Palindrome Strings	Greedy, Hash Table, String +1
1410	HTML Entity Parser	Hash Table, String
1418	Display Table of Food Orders in a Restaurant	Array, Hash Table, String +2
1419	Minimum Number of Frogs Croaking	String, Counting
1429	First Unique NumberPremium	Design, Queue, Array +2
1442	Count Triplets That Can Form Two Arrays of Equal XOR	Bit Manipulation, Array, Hash Table +2
1443	Minimum Time to Collect All Apples in a Tree	Tree, Depth-First Search, Breadth-First Search +1
1452	People Whose List of Favorite Companies Is Not a Subset of Another List	Array, Hash Table, String
1461	Check If a String Contains All Binary Codes of Size K	Bit Manipulation, Hash Table, String +2
1477	Find Two Non-overlapping Sub-arrays Each With Target Sum	Array, Hash Table, Binary Search +2
1481	Least Number of Unique Integers after K Removals	Greedy, Array, Hash Table +2
1485	Clone Binary Tree With Random PointerPremium	Tree, Depth-First Search, Breadth-First Search +2
1487	Making File Names Unique	Array, Hash Table, String
1488	Avoid Flood in The City	Greedy, Array, Hash Table +2
1490	Clone N-ary TreePremium	Tree, Depth-First Search, Breadth-First Search +1
1497	Check If Array Pairs Are Divisible by k	Array, Hash Table, Counting
1500	Design a File Sharing SystemPremium	Design, Hash Table, Data Stream +2
1506	Find Root of N-Ary TreePremium	Bit Manipulation, Tree, Depth-First Search +1
1519	Number of Nodes in the Sub-Tree With the Same Label	Tree, Depth-First Search, Breadth-First Search +2
1525	Number of Good Ways to Split a String	Bit Manipulation, Hash Table, String +1
1540	Can Convert String in K Moves	Hash Table, String
1545	Find Kth Bit in Nth Binary String	Recursion, String, Simulation
1546	Maximum Number of Non-Overlapping Subarrays With Sum Equals Target	Greedy, Array, Hash Table +1
1554	Strings Differ by One CharacterPremium	Hash Table, String, Hash Function +1
1562	Find Latest Group of Size M	Array, Hash Table, Binary Search +1
1570	Dot Product of Two Sparse VectorsPremium	Design, Array, Hash Table +1
1577	Number of Ways Where Square of Number Is Equal to Product of Two Numbers	Array, Hash Table, Math +1
1590	Make Sum Divisible by P	Array, Hash Table, Prefix Sum
1593	Split a String Into the Max Number of Unique Substrings	Hash Table, String, Backtracking
1600	Throne Inheritance	Tree, Depth-First Search, Design +1
1604	Alert Using Same Key-Card Three or More Times in a One Hour Period	Array, Hash Table, String +1
1612	Check If Two Expression Trees are EquivalentPremium	Tree, Depth-First Search, Hash Table +2
1630	Arithmetic Subarrays	Array, Hash Table, Sorting
1638	Count Substrings That Differ by One Character	Hash Table, String, Dynamic Programming +1
1647	Minimum Deletions to Make Character Frequencies Unique	Greedy, Hash Table, String +1
1650	Lowest Common Ancestor of a Binary Tree IIIPremium	Tree, Hash Table, Two Pointers +1
1658	Minimum Operations to Reduce X to Zero	Array, Hash Table, Binary Search +2
1660	Correct a Binary TreePremium	Tree, Depth-First Search, Breadth-First Search +2
1674	Minimum Moves to Make Array Complementary	Array, Hash Table, Prefix Sum
1676	Lowest Common Ancestor of a Binary Tree IVPremium	Tree, Depth-First Search, Hash Table +1
1695	Maximum Erasure Value	Array, Hash Table, Sliding Window
1711	Count Good Meals	Array, Hash Table
1726	Tuple with Same Product	Array, Hash Table, Counting
1733	Minimum Number of People to Teach	Greedy, Array, Hash Table
1737	Change Minimum Characters to Satisfy One of Three Conditions	Hash Table, String, Counting +1
1740	Find Distance in a Binary TreePremium	Tree, Depth-First Search, Breadth-First Search +2
1743	Restore the Array From Adjacent Pairs	Depth-First Search, Array, Hash Table
1772	Sort Features by PopularityPremium	Array, Hash Table, String +1
1775	Equal Sum Arrays With Minimum Number of Operations	Greedy, Array, Hash Table +1
1781	Sum of Beauty of All Substrings	Hash Table, String, Counting
1794	Count Pairs of Equal Substrings With Minimum DifferencePremium	Greedy, Hash Table, String
1797	Design Authentication Manager	Design, Hash Table, Linked List +1
1804	Implement Trie II (Prefix Tree)Premium	Design, Trie, Hash Table +1
1807	Evaluate the Bracket Pairs of a String	Array, Hash Table, String
1814	Count Nice Pairs in an Array	Array, Hash Table, Math +1
1817	Finding the Users Active Minutes	Array, Hash Table
1836	Remove Duplicates From an Unsorted Linked ListPremium	Hash Table, Linked List
1852	Distinct Numbers in Each SubarrayPremium	Array, Hash Table, Sliding Window
1865	Finding Pairs With a Certain Sum	Design, Array, Hash Table
1906	Minimum Absolute Difference Queries	Array, Hash Table
1915	Number of Wonderful Substrings	Bit Manipulation, Hash Table, String +1
1930	Unique Length-3 Palindromic Subsequences	Bit Manipulation, Hash Table, String +1
1940	Longest Common Subsequence Between Sorted ArraysPremium	Array, Hash Table, Counting
1942	The Number of the Smallest Unoccupied Chair	Array, Hash Table, Heap (Priority Queue)
1943	Describe the Painting	Array, Hash Table, Prefix Sum +1
1980	Find Unique Binary String	Array, Hash Table, String +1
1983	Widest Pair of Indices With Equal Range SumPremium	Array, Hash Table, Prefix Sum
1993	Operations on Tree	Tree, Depth-First Search, Breadth-First Search +3
2001	Number of Pairs of Interchangeable Rectangles	Array, Hash Table, Math +2
2007	Find Original Array From Doubled Array	Greedy, Array, Hash Table +1
2008	Maximum Earnings From Taxi	Array, Hash Table, Binary Search +2
2023	Number of Pairs of Strings With Concatenation Equal to Target	Array, Hash Table, String +1
2029	Stone Game IX	Greedy, Array, Math +2
2031	Count Subarrays With More Ones Than ZerosPremium	Binary Indexed Tree, Segment Tree, Array +5
2034	Stock Price Fluctuation	Design, Hash Table, Data Stream +2
2043	Simple Bank System	Design, Array, Hash Table +1
2048	Next Greater Numerically Balanced Number	Hash Table, Math, Backtracking +2
2067	Number of Equal Count SubstringsPremium	Hash Table, String, Counting +1
2075	Decode the Slanted Ciphertext	String, Simulation
2080	Range Frequency Queries	Design, Segment Tree, Array +2
2083	Substrings That Begin and End With the Same LetterPremium	Hash Table, Math, String +2
2107	Number of Unique Flavors After Sharing K CandiesPremium	Array, Hash Table, Sliding Window
2115	Find All Possible Recipes from Given Supplies	Graph, Topological Sort, Array +2
2120	Execution of All Suffix Instructions Staying in a Grid	String, Simulation
2121	Intervals Between Identical Elements	Array, Hash Table, Prefix Sum
2131	Longest Palindrome by Concatenating Two Letter Words	Greedy, Array, Hash Table +2
2135	Count Words Obtained After Adding a Letter	Bit Manipulation, Array, Hash Table +2
2150	Find All Lonely Numbers in the Array	Array, Hash Table, Counting
2152	Minimum Number of Lines to Cover PointsPremium	Bit Manipulation, Geometry, Array +5
2166	Design Bitset	Design, Array, Hash Table +1
2168	Unique Substrings With Equal Digit FrequencyPremium	Hash Table, String, Counting +2
2170	Minimum Operations to Make the Array Alternating	Greedy, Array, Hash Table +1
2182	Construct String With Repeat Limit	Greedy, Hash Table, String +2
2186	Minimum Number of Steps to Make Two Strings Anagram II	Hash Table, String, Counting
2196	Create Binary Tree From Descriptions	Tree, Array, Hash Table +1
2201	Count Artifacts That Can Be Extracted	Array, Hash Table, Simulation
2225	Find Players With Zero or One Losses	Array, Hash Table, Counting +1
2232	Minimize Result by Adding Parentheses to Expression	String, Enumeration
2244	Minimum Rounds to Complete All Tasks	Greedy, Array, Hash Table +1
2249	Count Lattice Points Inside a Circle	Geometry, Array, Hash Table +2
2250	Count Number of Rectangles Containing Each Point	Binary Indexed Tree, Array, Hash Table +2
2260	Minimum Consecutive Cards to Pick Up	Array, Hash Table, Sliding Window
2261	K Divisible Elements Subarrays	Trie, Array, Hash Table +3
2266	Count Number of Texts	Hash Table, Math, String +1
2268	Minimum Number of KeypressesPremium	Greedy, Hash Table, String +2
2275	Largest Combination With Bitwise AND Greater Than Zero	Bit Manipulation, Array, Hash Table +1
2284	Sender With Largest Word Count	Array, Hash Table, String +1
2288	Apply Discount to Prices	String
2295	Replace Elements in an Array	Array, Hash Table, Simulation
2342	Max Sum of a Pair With Equal Sum of Digits	Array, Hash Table, Sorting +1
2349	Design a Number Container System	Design, Hash Table, Ordered Set +1
2353	Design a Food Rating System	Design, Array, Hash Table +3
2364	Count Number of Bad Pairs	Array, Hash Table, Math +1
2365	Task Scheduler II	Array, Hash Table, Simulation
2368	Reachable Nodes With Restrictions	Tree, Depth-First Search, Breadth-First Search +4
2370	Longest Ideal Subsequence	Hash Table, String, Dynamic Programming
2374	Node With Highest Edge Score	Graph, Hash Table
2384	Largest Palindromic Number	Greedy, Hash Table, String +1
2385	Amount of Time for Binary Tree to Be Infected	Tree, Depth-First Search, Breadth-First Search +2
2405	Optimal Partition of String	Greedy, Hash Table, String
2408	Design SQLPremium	Design, Array, Hash Table +1
2414	Length of the Longest Alphabetical Continuous Substring	String
2424	Longest Uploaded Prefix	Union Find, Design, Binary Indexed Tree +5
2434	Using a Robot to Print the Lexicographically Smallest String	Stack, Greedy, Hash Table +1
2442	Count Number of Distinct Integers After Reverse Operations	Array, Hash Table, Math +1
2453	Destroy Sequential Targets	Array, Hash Table, Counting
2456	Most Popular Video Creator	Array, Hash Table, String +2
2461	Maximum Sum of Distinct Subarrays With Length K	Array, Hash Table, Sliding Window
2489	Number of Substrings With Fixed RatioPremium	Hash Table, Math, String +1
2491	Divide Players Into Teams of Equal Skill	Array, Hash Table, Two Pointers +1
2501	Longest Square Streak in an Array	Array, Hash Table, Binary Search +2
2502	Design Memory Allocator	Design, Array, Hash Table +1
2512	Reward Top K Students	Array, Hash Table, String +2
2516	Take K of Each Character From Left and Right	Hash Table, String, Sliding Window
2521	Distinct Prime Factors of Product of Array	Array, Hash Table, Math +1
2526	Find Consecutive Integers from a Data Stream	Design, Queue, Hash Table +2
2531	Make Number of Distinct Characters Equal	Hash Table, String, Counting
2537	Count the Number of Good Subarrays	Array, Hash Table, Sliding Window
2539	Count the Number of Good SubsequencesPremium	Hash Table, Math, String +2
2554	Maximum Number of Integers to Choose From a Range I	Greedy, Array, Hash Table +2
2564	Substring XOR Queries	Bit Manipulation, Array, Hash Table +1
2588	Count the Number of Beautiful Subarrays	Bit Manipulation, Array, Hash Table +1
2590	Design a Todo ListPremium	Design, Array, Hash Table +2
2593	Find Score of an Array After Marking All Elements	Array, Hash Table, Sorting +2
2597	The Number of Beautiful Subsets	Array, Hash Table, Math +4
2598	Smallest Missing Non-negative Integer After Operations	Greedy, Array, Hash Table +1
2606	Find the Substring With Maximum Cost	Array, Hash Table, String +1
2610	Convert an Array Into a 2D Array With Conditions	Array, Hash Table
2615	Sum of Distances	Array, Hash Table, Prefix Sum
2641	Cousins in Binary Tree II	Tree, Depth-First Search, Breadth-First Search +2
2653	Sliding Subarray Beauty	Array, Hash Table, Sliding Window
2657	Find the Prefix Common Array of Two Arrays	Bit Manipulation, Array, Hash Table
2661	First Completely Painted Row or Column	Array, Hash Table, Matrix
2671	Frequency Tracker	Design, Hash Table
2707	Extra Characters in a String	Trie, Array, Hash Table +2
2711	Difference of Number of Distinct Values on Diagonals	Array, Hash Table, Matrix
2718	Sum of Matrix After Queries	Array, Hash Table
2743	Count Substrings Without Repeating CharacterPremium	Hash Table, String, Sliding Window
2747	Count Zero Request Servers	Array, Hash Table, Sorting +1
2766	Relocate Marbles	Array, Hash Table, Sorting +1
2767	Partition String Into Minimum Beautiful Substrings	Hash Table, String, Dynamic Programming +1
2768	Number of Black Blocks	Array, Hash Table, Enumeration
2780	Minimum Index of a Valid Split	Array, Hash Table, Sorting
2782	Number of Unique CategoriesPremium	Union Find, Counting, Interactive
2799	Count Complete Subarrays in an Array	Array, Hash Table, Sliding Window
2808	Minimum Seconds to Equalize a Circular Array	Array, Hash Table
2830	Maximize the Profit as the Salesman	Array, Hash Table, Binary Search +2
2831	Find the Longest Equal Subarray	Array, Hash Table, Binary Search +1
2840	Check if Strings Can be Made Equal With Operations II	Hash Table, String, Sorting
2841	Maximum Sum of Almost Unique Subarray	Array, Hash Table, Sliding Window
2845	Count of Interesting Subarrays	Array, Hash Table, Prefix Sum
2852	Sum of Remoteness of All CellsPremium	Depth-First Search, Breadth-First Search, Union Find +3
2856	Minimum Array Length After Pair Removals	Greedy, Array, Hash Table +3
2857	Count Pairs of Points With Distance k	Bit Manipulation, Array, Hash Table
2870	Minimum Number of Operations to Make Array Empty	Greedy, Array, Hash Table +1
2875	Minimum Size Subarray in Infinite Array	Array, Hash Table, Prefix Sum +1
2898	Maximum Linear Stock ScorePremium	Array, Hash Table
2910	Minimum Number of Groups to Create a Valid Assignment	Greedy, Array, Hash Table
2914	Minimum Number of Changes to Make Binary String Beautiful	String
2933	High-Access Employees	Array, Hash Table, String +1
2947	Count Beautiful Substrings I	Hash Table, Math, String +3
2950	Number of Divisible SubstringsPremium	Hash Table, String, Counting +1
2955	Number of Same-End SubstringsPremium	Array, Hash Table, String +2
2958	Length of Longest Subarray With at Most K Frequency	Array, Hash Table, Sliding Window
2964	Number of Divisible Triplet SumsPremium	Array, Hash Table
2975	Maximum Square Area by Removing Fences From a Field	Array, Hash Table, Enumeration
2981	Find Longest Special Substring That Occurs Thrice I	Hash Table, String, Binary Search +2
2982	Find Longest Special Substring That Occurs Thrice II	Hash Table, String, Binary Search +2

Hard (110)

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
0065	Valid Number	String
0068	Text Justification	Array, String, Simulation
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
0214	Shortest Palindrome	String, String Matching, Hash Function +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
0248	Strobogrammatic Number IIIPremium	Recursion, Array, String
0305	Number of Islands IIPremium	Union Find, Array, Hash Table
0358	Rearrange String k Distance ApartPremium	Greedy, Hash Table, String +3
0548	Split Array with Equal SumPremium	Array, Hash Table, Prefix Sum
0588	Design In-Memory File SystemPremium	Design, Trie, Hash Table +2
0631	Design Excel Sum FormulaPremium	Graph, Design, Topological Sort +4
0711	Number of Distinct Islands IIPremium	Depth-First Search, Breadth-First Search, Union Find +2
0726	Number of Atoms	Stack, Hash Table, String +1
0761	Special Binary String	Recursion, String
0770	Basic Calculator IV	Stack, Recursion, Hash Table +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
1121	Divide Array Into Increasing SequencesPremium	Array, Counting
1153	String Transforms Into Another StringPremium	Graph, Hash Table, String
1172	Dinner Plate Stacks	Stack, Design, Hash Table +1
1178	Number of Valid Words for Each Puzzle	Bit Manipulation, Trie, Array +2
1224	Maximum Equal Frequency	Array, Hash Table
1255	Maximum Score Words Formed by Letters	Bit Manipulation, Array, Hash Table +5
1284	Minimum Number of Flips to Convert Binary Matrix to Zero Matrix	Bit Manipulation, Breadth-First Search, Array +2
1345	Jump Game IV	Breadth-First Search, Array, Hash Table
1392	Longest Happy Prefix	String, String Matching, Hash Function +1
1542	Find Longest Awesome Substring	Bit Manipulation, Hash Table, String
1713	Minimum Operations to Make a Subsequence	Greedy, Array, Hash Table +1
1782	Count Pairs Of Nodes	Graph, Array, Hash Table +4
1788	Maximize the Beauty of the GardenPremium	Greedy, Array, Hash Table +1
1819	Number of Different Subsequences GCDs	Array, Math, Counting +1
1857	Largest Color Value in a Directed Graph	Graph, Topological Sort, Memoization +3
1912	Design Movie Rental System	Design, Array, Hash Table +2
1932	Merge BSTs to Create Single BST	Tree, Depth-First Search, Hash Table +2
1938	Maximum Genetic Difference Query	Bit Manipulation, Depth-First Search, Trie +2
1948	Delete Duplicate Folders in System	Trie, Array, Hash Table +2
1960	Maximum Product of the Length of Two Palindromic Substrings	String, Hash Function, Rolling Hash
1994	The Number of Good Subsets	Bit Manipulation, Array, Hash Table +5
2009	Minimum Number of Operations to Make Array Continuous	Array, Hash Table, Binary Search +1
2014	Longest Subsequence Repeated k Times	Greedy, String, Backtracking +2
2019	The Score of Students Solving Math Expression	Stack, Memoization, Array +4
2025	Maximum Number of Ways to Partition an Array	Array, Hash Table, Counting +2
2122	Recover the Original Array	Array, Hash Table, Two Pointers +2
2213	Longest Substring of One Repeating Character	Segment Tree, Array, String +1
2227	Encrypt and Decrypt Strings	Design, Trie, Array +2
2251	Number of Flowers in Full Bloom	Array, Hash Table, Binary Search +3
2254	Design Video Sharing PlatformPremium	Stack, Design, Hash Table +1
2262	Total Appeal of A String	Hash Table, String, Dynamic Programming
2301	Match Substring After Replacement	Array, Hash Table, String +1
2306	Naming a Company	Bit Manipulation, Array, Hash Table +2
2350	Shortest Impossible Sequence of Rolls	Greedy, Array, Hash Table
2354	Number of Excellent Pairs	Bit Manipulation, Array, Hash Table +1
2402	Meeting Rooms III	Array, Hash Table, Sorting +2
2416	Sum of Prefix Scores of Strings	Trie, Array, String +1
2421	Number of Good Paths	Tree, Union Find, Graph +3
2488	Count Subarrays With Median K	Array, Hash Table, Prefix Sum
2499	Minimum Total Cost to Make Arrays Unequal	Greedy, Array, Hash Table +1
2508	Add Edges to Make Degrees of All Nodes Even	Graph, Hash Table
2514	Count Anagrams	Hash Table, Math, String +2
2524	Maximum Frequency Score of a SubarrayPremium	Stack, Array, Hash Table +2
2547	Minimum Cost to Split an Array	Array, Hash Table, Dynamic Programming +1
2561	Rearranging Fruits	Greedy, Sort, Array +1
2581	Count Number of Possible Root Nodes	Tree, Depth-First Search, Array +2
2584	Split the Array to Make Coprime Products	Array, Hash Table, Math +1
2612	Minimum Reverse Operations	Breadth-First Search, Union Find, Array +2
2713	Maximum Strictly Increasing Cells in a Matrix	Memoization, Array, Hash Table +5
2732	Find a Good Subset of the Matrix	Bit Manipulation, Array, Hash Table +1
2763	Sum of Imbalance Numbers of All Subarrays	Array, Hash Table, Enumeration
2781	Length of the Longest Valid Substring	Array, Hash Table, String +1
2813	Maximum Elegance of a K-Length Subsequence	Stack, Greedy, Array +3
2842	Count K-Subsequences of a String With Maximum Beauty	Greedy, Hash Table, Math +2
2897	Apply Operations on Array to Maximize Sum of Squares	Greedy, Bit Manipulation, Array +1
2902	Count of Sub-Multisets With Bounded Sum	Array, Hash Table, Dynamic Programming +1
2935	Maximum Strong Pair XOR II	Bit Manipulation, Trie, Array +2
2949	Count Beautiful Substrings II	Hash Table, Math, String +2
2953	Count Complete Substrings	Hash Table, String, Sliding Window
2963	Count the Number of Good Partitions	Array, Hash Table, Math +1
2983	Palindrome Rearrangement Queries	Hash Table, String, Prefix Sum

Hash Map pattern FAQ

What is the hash map pattern?

A hash map converts a search into a lookup, which is why it is the most common single answer in this entire corpus — 588 of the 3000 problems are built on one.

How many LeetCode problems use the hash map pattern?

This page lists 709 LeetCode problems that the hash map pattern applies to: 227 Easy, 372 Medium and 110 Hard. 601 of them carry a complete Python solution with complexity analysis.

What is the time complexity of the hash map pattern?

O(1) average per operation, O(n) worst case time and O(n) space. Average-case constant lookups are what turn a nested-loop search into a single pass, so the complement and frequency-counting solutions are O(n) time and O(n) space. The worst case is linear per operation when every key collides, which is irrelevant on real data and is the reason a language's string hash is randomised per process. Ordering is not free: getting a sorted result out of a map costs the sort on top.

When should I use the hash map pattern in an interview?

The brute force is a nested loop asking "is there another element such that…". You are counting occurrences, or comparing two multisets.

Which hash map problem should I start with?

LeetCode 1. Two Sum is the lowest-numbered Easy problem on this page, which makes it the usual starting point: the technique is visible without the problem's own complications getting in the way.

What patterns are related to hash map?

Prefix Sum, Sliding Window, Sorting, Trie. Problems frequently sit in more than one of these, and the overlap is where the interesting follow-up questions come from.

More ways in: all 22 patterns, the curated study lists, or the full problem list.

What the hash map pattern is

When to use it

The hash map template in Python

Complexity characteristics

Meet the hash map problem you did not practise.

Approach

Approach

Approach

All 709 hash map LeetCode problems

Easy (227)227

Medium (372)372

Hard (110)110

Related patterns

Hash Map pattern FAQ

What is the hash map pattern?

How many LeetCode problems use the hash map pattern?

What is the time complexity of the hash map pattern?

When should I use the hash map pattern in an interview?

Which hash map problem should I start with?

What patterns are related to hash map?

Easy (227)

Medium (372)

Hard (110)