Hash Table Interview Questions

O(1) lookups, frequency maps, and set-based reasoning.

260 problems

#	Title	Difficulty	Topics
1	Two Sum	Easy	hash table
3	Longest Substring Without Repeating Characters	Medium	hash table
30	Substring with Concatenation of All Words	Hard	hash table
36	Valid Sudoku	Easy	hash table
49	Group Anagrams	Medium	hash table
76	Minimum Window Substring	Hard	hash table
149	Max Points on a Line	Hard	hash table
159	Longest Substring with At Most Two Distinct Characters	Hard	hash table
170	Two Sum III - Data structure design	Easy	hash table
187	Repeated DNA Sequences	Medium	hash table
202	Happy Number	Easy	hash table
204	Count Primes	Easy	hash table
205	Isomorphic Strings	Easy	hash table
217	Contains Duplicate	Easy	hash table
219	Contains Duplicate II	Easy	hash table
244	Shortest Word Distance II	Medium	hash table
246	Strobogrammatic Number	Easy	hash table
249	Group Shifted Strings	Easy	hash table
266	Palindrome Permutation	Easy	hash table
288	Unique Word Abbreviation	Easy	hash table
290	Word Pattern	Easy	hash table
299	Bulls and Cows	Easy	hash table
305	Number of Islands II	Hard	hash table
314	Binary Tree Vertical Order Traversal	Medium	hash table
323	Number of Connected Components in an Undirected Graph	Medium	hash table
325	Maximum Size Subarray Sum Equals k	Medium	hash table
336	Palindrome Pairs	Hard	hash table
340	Longest Substring with At Most K Distinct Characters	Hard	hash table
356	Line Reflection	Medium	hash table
387	First Unique Character in a String	Easy	hash table
388	Longest Absolute File Path	Medium	hash table
409	Longest Palindrome	Easy	hash table
438	Find All Anagrams in a String	Easy	hash table
447	Number of Boomerangs	Easy	hash table
454	4Sum II	Medium	hash table
473	Matchsticks to Square	Medium	hash table
523	Continuous Subarray Sum	Medium	hash table
525	Contiguous Array	Medium	hash table
532	K-diff Pairs in an Array	Easy	hash table
554	Brick Wall	Medium	hash table
560	Subarray Sum Equals K	Medium	hash table
561	Array Partition I	Easy	hash table
575	Distribute Candies	Easy	hash table
594	Longest Harmonious Subsequence	Easy	hash table
599	Minimum Index Sum of Two Lists	Easy	hash table
609	Find Duplicate File in System	Medium	hash table
721	Accounts Merge	Medium	hash table
734	Sentence Similarity	Easy	hash table
737	Sentence Similarity II	Medium	hash table
748	Shortest Completing Word	Easy	hash table
760	Find Anagram Mappings	Easy	hash table
771	Jewels and Stones	Easy	hash table
811	Subdomain Visit Count	Easy	hash table
822	Card Flipping Game	Medium	hash table
825	Friends Of Appropriate Ages	Medium	hash table
869	Reordered Power of 2	Medium	hash table
873	Length of Longest Fibonacci Subsequence	Medium	hash table
957	Prison Cells After N Days	Medium	hash table
966	Vowel Spellchecker	Medium	hash table
974	Subarray Sums Divisible by K	Medium	hash table
982	Triples with Bitwise AND Equal To Zero	Hard	hash table
1001	Grid Illumination	Hard	hash table
1124	Longest Well-Performing Interval	Medium	hash table
1133	Largest Unique Number	Easy	hash table
1152	Analyze User Website Visit Pattern	Medium	hash table
1153	String Transforms Into Another String	Hard	hash table
1160	Find Words That Can Be Formed by Characters	Easy	hash table
1165	Single-Row Keyboard	Easy	hash table
1198	Find Smallest Common Element in All Rows	Medium	hash table
1207	Unique Number of Occurrences	Easy	hash table
1224	Maximum Equal Frequency	Hard	hash table
1418	Display Table of Food Orders in a Restaurant	Medium	hash table
1452	People Whose List of Favorite Companies Is Not a Subset of Another List	Medium	hash table
1487	Making File Names Unique	Medium	hash table
1577	Number of Ways Where Square of Number Is Equal to Product of Two Numbers	Medium	hash table
1590	Make Sum Divisible by P	Medium	hash table
1640	Check Array Formation Through Concatenation	Easy	hash table
1657	Determine if Two Strings Are Close	Medium	hash table
1679	Max Number of K-Sum Pairs	Medium	hash table
1711	Count Good Meals	Medium	hash table
1748	Sum of Unique Elements	Easy	hash table
1781	Sum of Beauty of All Substrings	Medium	hash table
1807	Evaluate the Bracket Pairs of a String	Medium	hash table
1814	Count Nice Pairs in an Array	Medium	hash table
1817	Finding the Users Active Minutes	Medium	hash table
1915	Number of Wonderful Substrings	Medium	hash table
1923	Longest Common Subpath	Hard	hash table
1925	Count Square Sum Triples	Easy	hash table
1930	Unique Length-3 Palindromic Subsequences	Medium	hash table
1941	Check if All Characters Have Equal Number of Occurrences	Easy	hash table
1995	Count Special Quadruplets	Easy	hash table
2006	Count Number of Pairs With Absolute Difference K	Easy	hash table
2023	Number of Pairs of Strings With Concatenation Equal to Target	Medium	hash table
2025	Maximum Number of Ways to Partition an Array	Hard	hash table
2032	Two Out of Three	Easy	hash table
2053	Kth Distinct String in an Array	Easy	hash table
2068	Check Whether Two Strings are Almost Equivalent	Easy	hash table
2085	Count Common Words With One Occurrence	Easy	hash table
2120	Execution of All Suffix Instructions Staying in a Grid	Medium	hash table
2150	Find All Lonely Numbers in the Array	Medium	hash table
2154	Keep Multiplying Found Values by Two	Easy	hash table
2170	Minimum Operations to Make the Array Alternating	Medium	hash table
2190	Most Frequent Number Following Key In an Array	Easy	hash table
2201	Count Artifacts That Can Be Extracted	Medium	hash table
2206	Divide Array Into Equal Pairs	Easy	hash table
2215	Find the Difference of Two Arrays	Easy	hash table
2225	Find Players With Zero or One Losses	Medium	hash table
2229	Check if an Array Is Consecutive	Easy	hash table
2260	Minimum Consecutive Cards to Pick Up	Medium	hash table
2261	K Divisible Elements Subarrays	Medium	hash table
2283	Check if Number Has Equal Digit Count and Digit Value	Easy	hash table
2284	Sender With Largest Word Count	Medium	hash table
2287	Rearrange Characters to Make Target String	Easy	hash table
2295	Replace Elements in an Array	Medium	hash table
2306	Naming a Company	Hard	hash table
2309	Greatest English Letter in Upper and Lower Case	Easy	hash table
2325	Decode the Message	Easy	hash table
2341	Maximum Number of Pairs in Array	Easy	hash table
2342	Max Sum of a Pair With Equal Sum of Digits	Medium	hash table
2347	Best Poker Hand	Easy	hash table
2351	First Letter to Appear Twice	Easy	hash table
2352	Equal Row and Column Pairs	Medium	hash table
2354	Number of Excellent Pairs	Hard	hash table
2357	Make Array Zero by Subtracting Equal Amounts	Easy	hash table
2363	Merge Similar Items	Easy	hash table
2364	Count Number of Bad Pairs	Medium	hash table
2365	Task Scheduler II	Medium	hash table
2367	Number of Arithmetic Triplets	Easy	hash table
2374	Node With Highest Edge Score	Medium	hash table
2395	Find Subarrays With Equal Sum	Easy	hash table
2399	Check Distances Between Same Letters	Easy	hash table
2404	Most Frequent Even Element	Easy	hash table
2423	Remove Letter To Equalize Frequency	Easy	hash table
2441	Largest Positive Integer That Exists With Its Negative	Easy	hash table
2442	Count Number of Distinct Integers After Reverse Operations	Medium	hash table
2451	Odd String Difference	Easy	hash table
2452	Words Within Two Edits of Dictionary	Medium	hash table
2453	Destroy Sequential Targets	Medium	hash table
2456	Most Popular Video Creator	Medium	hash table
2484	Count Palindromic Subsequences	Hard	hash table
2488	Count Subarrays With Median K	Hard	hash table
2489	Number of Substrings With Fixed Ratio	Medium	hash table
2491	Divide Players Into Teams of Equal Skill	Medium	hash table
2501	Longest Square Streak in an Array	Medium	hash table
2506	Count Pairs Of Similar Strings	Easy	hash table
2531	Make Number of Distinct Characters Equal	Medium	hash table
2564	Substring XOR Queries	Medium	hash table
2588	Count the Number of Beautiful Subarrays	Medium	hash table
2592	Maximize Greatness of an Array	Medium	hash table
2598	Smallest Missing Non-negative Integer After Operations	Medium	hash table
2605	Form Smallest Number From Two Digit Arrays	Easy	hash table
2615	Sum of Distances	Medium	hash table
2657	Find the Prefix Common Array of Two Arrays	Medium	hash table
2661	First Completely Painted Row or Column	Medium	hash table
2670	Find the Distinct Difference Array	Easy	hash table
2716	Minimize String Length	Easy	hash table
2718	Sum of Matrix After Queries	Medium	hash table
2744	Find Maximum Number of String Pairs	Easy	hash table
2748	Number of Beautiful Pairs	Easy	hash table
2766	Relocate Marbles	Medium	hash table
2768	Number of Black Blocks	Medium	hash table
2784	Check if Array is Good	Easy	hash table
2808	Minimum Seconds to Equalize a Circular Array	Medium	hash table
2815	Max Pair Sum in an Array	Easy	hash table
2839	Check if Strings Can be Made Equal With Operations I	Easy	hash table
2840	Check if Strings Can be Made Equal With Operations II	Medium	hash table
2845	Count of Interesting Subarrays	Medium	hash table
2857	Count Pairs of Points With Distance k	Medium	hash table
2869	Minimum Operations to Collect Elements	Easy	hash table
2943	Maximize Area of Square Hole in Grid	Medium	hash table
2947	Count Beautiful Substrings I	Medium	hash table
2949	Count Beautiful Substrings II	Hard	hash table
2950	Number of Divisible Substrings	Medium	hash table
2955	Number of Same-End Substrings	Medium	hash table
2956	Find Common Elements Between Two Arrays	Easy	hash table
2964	Number of Divisible Triplet Sums	Medium	hash table
2975	Maximum Square Area by Removing Fences From a Field	Medium	hash table
2981	Find Longest Special Substring That Occurs Thrice I	Medium	hash table
2982	Find Longest Special Substring That Occurs Thrice II	Medium	hash table
2983	Palindrome Rearrangement Queries	Hard	hash table
2996	Smallest Missing Integer Greater Than Sequential Prefix Sum	Easy	hash table
3005	Count Elements With Maximum Frequency	Easy	hash table
3039	Apply Operations to Make String Empty	Medium	hash table
3044	Most Frequent Prime	Medium	hash table
3046	Split the Array	Easy	hash table
3078	Match Alphanumerical Pattern in Matrix I	Medium	hash table
3083	Existence of a Substring in a String and Its Reverse	Easy	hash table
3120	Count the Number of Special Characters I	Easy	hash table
3121	Count the Number of Special Characters II	Medium	hash table
3137	Minimum Number of Operations to Make Word K-Periodic	Medium	hash table
3138	Minimum Length of Anagram Concatenation	Medium	hash table
3143	Maximum Points Inside the Square	Medium	hash table
3146	Permutation Difference between Two Strings	Easy	hash table
3158	Find the XOR of Numbers Which Appear Twice	Easy	hash table
3160	Find the Number of Distinct Colors Among the Balls	Medium	hash table
3167	Better Compression of String	Medium	hash table
3184	Count Pairs That Form a Complete Day I	Easy	hash table
3185	Count Pairs That Form a Complete Day II	Medium	hash table
3223	Minimum Length of String After Operations	Medium	hash table
3237	Alt and Tab Simulation	Medium	hash table
3238	Find the Number of Winning Players	Easy	hash table
3245	Alternating Groups III	Hard	hash table
3295	Report Spam Message	Medium	hash table
3365	Rearrange K Substrings to Form Target String	Medium	hash table
3371	Identify the Largest Outlier in an Array	Medium	hash table
3396	Minimum Number of Operations to Make Elements in Array Distinct	Easy	hash table
3404	Count Special Subsequences	Medium	hash table
3438	Find Valid Pair of Adjacent Digits in String	Easy	hash table
3442	Maximum Difference Between Even and Odd Frequency I	Easy	hash table
3450	Maximum Students on a Single Bench	Easy	hash table
3471	Find the Largest Almost Missing Integer	Easy	hash table
3487	Maximum Unique Subarray Sum After Deletion	Easy	hash table
3488	Closest Equal Element Queries	Medium	hash table
3527	Find the Most Common Response	Medium	hash table
3541	Find Most Frequent Vowel and Consonant	Easy	hash table
3572	Maximize Y‑Sum by Picking a Triplet of Distinct X‑Values	Medium	hash table
3581	Count Odd Letters from Number	Easy	hash table
3591	Check if Any Element Has Prime Frequency	Easy	hash table
3636	Threshold Majority Queries	Hard	hash table
3662	Filter Characters by Frequency	Easy	hash table
3663	Find The Least Frequent Digit	Easy	hash table
3668	Restore Finishing Order	Easy	hash table
3678	Smallest Absent Positive Greater Than Average	Easy	hash table
3682	Minimum Index Sum of Common Elements	Medium	hash table
3692	Majority Frequency Characters	Easy	hash table
3694	Distinct Points Reachable After Substring Removal	Medium	hash table
3712	Sum of Elements With Frequency Divisible by K	Easy	hash table
3713	Longest Balanced Substring I	Medium	hash table
3714	Longest Balanced Substring II	Medium	hash table
3718	Smallest Missing Multiple of K	Easy	hash table
3728	Stable Subarrays With Equal Boundary and Interior Sum	Medium	hash table
3729	Count Distinct Subarrays Divisible by K in Sorted Array	Hard	hash table
3755	Find Maximum Balanced XOR Subarray Length	Medium	hash table
3760	Maximum Substrings With Distinct Start	Medium	hash table
3761	Minimum Absolute Distance Between Mirror Pairs	Medium	hash table
3773	Maximum Number of Equal Length Runs	Medium	hash table
3779	Minimum Number of Operations to Have Distinct Elements	Medium	hash table
3784	Minimum Deletion Cost to Make All Characters Equal	Medium	hash table
3803	Count Residue Prefixes	Easy	hash table
3804	Number of Centered Subarrays	Medium	hash table
3805	Count Caesar Cipher Pairs	Medium	hash table
3810	Minimum Operations to Reach Target Array	Medium	hash table
3837	Delayed Count of Equal Elements	Medium	hash table
3839	Number of Prefix Connected Groups	Medium	hash table
3842	Toggle Light Bulbs	Easy	hash table
3843	First Element with Unique Frequency	Medium	hash table
3846	Total Distance to Type a String Using One Finger	Medium	hash table
3848	Check Digitorial Permutation	Medium	hash table
3852	Smallest Pair With Different Frequencies	Easy	hash table
3866	First Unique Even Element	Easy	hash table
3889	Mirror Frequency Distance	Medium	hash table
3890	Integers With Multiple Sum of Two Cubes	Medium	hash table
3900	Longest Balanced Substring After One Swap	Medium	hash table
3901	Good Subsequence Queries	Medium	hash table
3913	Sort Vowels by Frequency	Medium	hash table
3917	Count Indices With Opposite Parity	Easy	hash table
3921	Score Validator	Easy	hash table
3926	Count Valid Word Occurrences	Medium	hash table
3945	Digit Frequency Score	Easy	hash table
3960	Frequency Balance Subarray	Medium	hash table