Hash Map and Hashing coding problems

Hash maps turn slow nested loops into one pass. Practise frequency counting, seen-sets, grouping and the classic two-sum family, the patterns interviewers expect you to reach for first.

101 problems: 26 easy, 70 medium, 5 hard. Free to read; a free account solves them and earns XP and Aura.

1. Count Character OccurrencesPractice · +10 XP · strings, loops, countingeasy
2. Count Distinct ElementsPractice · +10 XP · sorting, counting, arrayseasy
3. Count Element OccurrencesPractice · +10 XP · hash map, counting, frequencyeasy
4. Count Elements Greater Than KPractice · +10 XP · hash map, counting, conditionalseasy
5. Count Elements Greater Than NeighborPractice · +10 XP · arrays, countingeasy
6. Count Elements Less Than K in WindowPractice · +10 XP · sliding window, countingeasy
7. Count Non-Alphabetic CharactersPractice · +10 XP · strings, counting, loopseasy
8. Count Number OccurrencesPractice · +10 XP · hash map, counting, frequencyeasy
9. Count Numbers with Even Set BitsPractice · +10 XP · bit manipulation, loops, countingeasy
10. Count Occurrences of a Target ValuePractice · +10 XP · arrays, loops, countingeasy
11. Count Occurrences of Each NumberPractice · +10 XP · hash map, counting, frequencyeasy
12. Count Pairs with Smaller SumPractice · +10 XP · two pointers, sorting, countingeasy
13. Count Paths on a LinePractice · +10 XP · dynamic programming, recursion, countingeasy
14. Count Specific CharactersPractice · +10 XP · strings, counting, loopseasy
15. Count Specific ElementsPractice · +10 XP · hash map, counting, frequencyeasy
16. Count Subarrays with Target SumPractice · +10 XP · sliding window, prefix sums, hash mapeasy
17. Count Subsequences Summing to KPractice · +10 XP · dynamic programming, recursion, countingeasy
18. Count Uppercase LettersPractice · +10 XP · strings, loops, countingeasy
19. Count Value OccurrencesPractice · +10 XP · hash map, counting, frequencyeasy
20. Count Vowel PairsPractice · +10 XP · strings, loops, countingeasy
21. Majority ElementDaily · +5 XP · +3 Aura (late, half) · arrays, counting2 solvedeasy
22. Most Frequent CharacterPractice · +10 XP · hash map, strings, countingeasy
23. Most Frequent ElementPractice · +10 XP · hash map, counting, frequencyeasy
24. Number of Students per CollegePractice · SQL · +10 XP · group by, sql, countingeasy
25. String AnalyzerPractice · +10 XP · strings, counting, simple parsingeasy
26. The Polite WaiterDaily · +5 XP · +3 Aura (late, half) · strings, counting, loops1 solvedeasy
27. Array of SubarraysDaily · +15 XP · +5 Aura (late, half) · hash map, two pointers, subarray2 solvedmedium
28. Consecutive DuplicatesPractice · +20 XP · arrays, counting, run-length encodingmedium
29. Count Distinct Prefix SumsPractice · +20 XP · math, prefix sums, hashingmedium
30. Count Distinct ValuesPractice · +20 XP · hash map, counting, frequencymedium
31. Count Elements with Equal NeighborsPractice · +20 XP · hash map, counting, arraysmedium
32. Count Matching PairsPractice · +20 XP · hash map, counting, arraysmedium
33. Count Pairs with Digit Sum Divisible by KPractice · +20 XP · math, hashing, prefix sumsmedium
34. Count Pairs with Equal Digit ProductPractice · +20 XP · math, hashing, number theorymedium
35. Count Pairs with Equal Digit SumPractice · +20 XP · math, hashing, arraysmedium
36. Count Pairs with Equal ProductPractice · +20 XP · arrays, hash map, countingmedium
37. Count Pairs with Equal SumPractice · +20 XP · hash map, countingmedium
38. Count Pairs with Equal XOR SumPractice · +20 XP · recursion, prefix sums, hash mapsmedium
39. Count Pairs with Same RemainderPractice · +20 XP · hash map, counting, modulo arithmeticmedium
40. Count Pairs with Sum Divisible by KPractice · +20 XP · arrays, hashing, number theorymedium
41. Count Subarrays with Distinct ElementsPractice · +20 XP · hash map, sliding window, two pointersmedium
42. Count Subarrays with Equal Prefix SumsPractice · +20 XP · prefix sums, hash map, countingmedium
43. Count Subarrays with Target Sum DifferencePractice · +20 XP · prefix sums, hash map, subarray sumsmedium
44. Count Uncommon ElementsPractice · +20 XP · hash map, countingmedium
45. Count Unique Element PairsPractice · +20 XP · hash map, two pointers, sortingmedium
46. Count XOR PairsPractice · +20 XP · bit manipulation, hash map, prefix sumsmedium
47. Distinct Consecutive PairsPractice · +20 XP · arrays, hash map, countingmedium
48. Distinct Element Sum with Minimum CostPractice · +20 XP · heaps, hash map, greedymedium
49. Distinct Elements CountPractice · +20 XP · greedy, hash map, countingmedium
50. Distinct Elements in a Sliding WindowPractice · +20 XP · arrays, hash map, sliding windowmedium
51. Distinct Elements in PrefixPractice · +20 XP · arrays, hash set, prefix sumsmedium
52. Distinct Elements in Sliding WindowPractice · +20 XP · stack, hash map, sliding windowmedium
53. Distinct Elements with Minimum Total CostPractice · +20 XP · heaps, hash set, greedymedium
54. Distinct Row CountsPractice · +20 XP · matrix, hashing, data structuresmedium
55. Distinct Subgrid ValuesPractice · +20 XP · matrix, prefix sums, hash setmedium
56. Distinct XOR Prefix SumsPractice · +20 XP · bit manipulation, prefix sums, hash setmedium
57. Divisible by Last DigitPractice · +20 XP · math, number theory, hash mapmedium
58. Find Duplicate Cycle EntryPractice · +20 XP · linked structures, hash map, cycle detectionmedium
59. Find Duplicate Jump TargetPractice · +20 XP · linked structures, hash map, simulationmedium
60. Find K Distinct Element FrequenciesPractice · +20 XP · heaps, frequency counting, top-kmedium
61. Find the First Unique ElementPractice · +20 XP · hash map, arrays, frequencymedium
62. Group by PropertyPractice · +20 XP · hash map, grouping, sortingmedium
63. Group by ValuePractice · +20 XP · hash map, grouping, arraysmedium
64. Linked List CompressionPractice · +20 XP · linked structures, hash maps, simulationmedium
65. Linked List Path SumPractice · +20 XP · linked structures, hash map, cycle detectionmedium
66. Longest Common SuffixPractice · +20 XP · strings, hashing, suffix arraymedium
67. Longest Prefix with Equal CountsPractice · +20 XP · stack, hash map, prefix summedium
68. Longest Prefix With No RepeatsPractice · +20 XP · strings, sliding window, hash setmedium
69. Longest Segment with At Most K Distinct ElementsPractice · +20 XP · sliding window, hash map, frequency countingmedium
70. Longest Subarray with At Most K Distinct ElementsPractice · +20 XP · two pointers, sliding window, hash mapmedium
71. Longest Subarray with At Most K Duplicate PairsPractice · +20 XP · sliding window, hash map, arraysmedium
72. Longest Subarray with Equal HalvesPractice · +20 XP · sliding window, prefix sums, hash mapmedium
73. Longest Subarray With Sum XPractice · +20 XP · sliding window, prefix sums, hash mapmedium
74. Longest Substring With At Most K Distinct CharactersPractice · +20 XP · strings, sliding window, hash mapmedium
75. Longest Substring Without RepeatsPractice · +20 XP · sliding window, hash map, stringsmedium
76. Matrix Neighbors SumPractice · +20 XP · matrix, bfs, hash mapmedium
77. Matrix Zero Sum SubgridsPractice · +20 XP · matrix, prefix sums, hash mapmedium
78. Minimum XOR Sum of SubarraysPractice · +20 XP · bit manipulation, prefix sums, hash mapmedium
79. Most Frequent Row XORPractice · +20 XP · matrix, hash map, frequencymedium
80. Most Frequent SubstringPractice · +20 XP · strings, hashing, countingmedium
81. Pairs with Sum Divisible by KPractice · +20 XP · math, number theory, hash mapmedium
82. Sort by Subsequence SumPractice · +20 XP · sorting, hash map, prefix summedium
83. String PartitionPractice · +20 XP · strings, hashing, prefix sumsmedium
84. String with Most Unique CharactersPractice · +20 XP · strings, sliding window, hash mapmedium
85. Subarray Sum and CountPractice · +20 XP · prefix sums, hash map, range queriesmedium
86. Subarray Sum FrequenciesPractice · +20 XP · prefix sums, hash map, subarray sumsmedium
87. Subarray Sum MultiplesPractice · +20 XP · prefix sums, hash maps, number theorymedium
88. Subarray Sums with Target DifferencePractice · +20 XP · prefix sums, hash maps, countingmedium
89. Subarray with Target ProductPractice · +20 XP · arrays, hash map, prefix productmedium
90. Subarray XOR with TargetPractice · +20 XP · bit manipulation, hash map, prefix summedium
91. Subsegment Sum Divisible by KPractice · +20 XP · recursion, prefix sums, hashingmedium
92. Sum of Distinct ElementsPractice · +20 XP · recursion, hashing, arraysmedium
93. Tree Subtree SumsPractice · +20 XP · trees, dfs, hash mapmedium
94. Two Sum Modulo KPractice · +20 XP · math, hashing, number theorymedium
95. XOR Pairs with Zero SumPractice · +20 XP · bit manipulation, hash map, prefix sumsmedium
96. XOR Sequence PartitionPractice · +20 XP · bit manipulation, hash map, prefix sumsmedium
97. Distinct Subarrays with Bounded SumPractice · +30 XP · two pointers, hash map, combinatoricshard
98. Distinct Subsequences CountPractice · +30 XP · hash map, dynamic programming, stringhard
99. Longest Balanced SubarrayPractice · +30 XP · arrays, hash maps, prefix sumshard
100. Longest Subarray with Distinct ElementsPractice · +30 XP · hash map, sliding window, two pointershard
101. Longest Subarray with Equal Number of Zeroes and OnesPractice · +30 XP · two pointers, prefix sum, hash maphard