Arrays coding problems

Array problems are the first round of almost every coding interview and online assessment. Practise indexing, in-place updates, rotations and scans, then move on to prefix sums, two pointers and sliding windows built on top of them.

88 problems: 51 easy, 32 medium, 5 hard. Free to read; a free account solves them and earns XP and Aura.

1. Count Consecutive OccurrencesPractice · +10 XP · dynamic programming, arrays, loopseasy
2. Count Distinct ElementsPractice · +10 XP · sorting, counting, arrayseasy
3. Count Elements Greater Than NeighborPractice · +10 XP · arrays, countingeasy
4. Count Elements Greater Than PreviousPractice · +10 XP · greedy, arrays, loopseasy
5. Count Elements Less Than KPractice · +10 XP · binary search, arrayseasy
6. Count Equal NeighborsPractice · +10 XP · basics, arrays, loopseasy
7. Count Even Numbers in WindowPractice · +10 XP · sliding window, arrayseasy
8. Count Max Element in WindowPractice · +10 XP · sliding window, arrayseasy
9. Count Non-Negative SubarraysPractice · +10 XP · sliding window, prefix sums, arrayseasy
10. Count Numbers Divisible by KPractice · +10 XP · dynamic programming, arrays, basic programmingeasy
11. Count occurrences of a numberPractice · +10 XP · basics, loops, arrayseasy
12. Count Occurrences of a Target ValuePractice · +10 XP · arrays, loops, countingeasy
13. Count Pairs with Specific SumPractice · +10 XP · sliding window, two pointers, arrayseasy
14. Find Duplicate in Sorted ArrayPractice · +10 XP · two pointers, arrays, binary searcheasy
15. Find Element After StepsPractice · +10 XP · linked structures, simulation, arrayseasy
16. Find Element in Sorted ArrayPractice · +10 XP · binary search, arrayseasy
17. Find End of Linked ListPractice · +10 XP · linked structures, simulation, arrayseasy
18. Find First Even NumberPractice · +10 XP · two pointers, arrays, searchingeasy
19. Find First OccurrencePractice · +10 XP · binary search, arrays, searchingeasy
20. Find Linked List TailPractice · +10 XP · linked structures, arrays, traversaleasy
21. Find Max Element in WindowPractice · +10 XP · sliding window, arrays, maximumeasy
22. Find Minimum and MaximumPractice · +10 XP · arrays, min-maxeasy
23. Find Node at Fixed DepthPractice · +10 XP · linked structures, simulation, arrayseasy
24. Find Pair with SumPractice · +10 XP · two pointers, array, two sumeasy
25. Find Root of TreePractice · +10 XP · trees, arrayseasy
26. Find Second Largest ElementPractice · +10 XP · arrays, sorting, loopseasy
27. Find Smallest Missing Positive IntegerPractice · +10 XP · two pointers, binary search, arrayseasy
28. Find the First Negative NumberPractice · +10 XP · queue, simulation, arrayseasy
29. Find the Single NumberPractice · +10 XP · bit manipulation, xor tricks, arrayseasy
30. Last Element in RangePractice · +10 XP · stack, arrayseasy
31. Leaf CountPractice · +10 XP · trees, arrayseasy
32. Majority ElementDaily · +5 XP · +3 Aura (late, half) · arrays, counting2 solvedeasy
33. Max Element in PrefixPractice · +10 XP · stack, prefix sums, arrayseasy
34. Maximize Non-Negative SumPractice · +10 XP · greedy, arrayseasy
35. Maximum Sum of a Fixed-Size SubarrayPractice · +10 XP · sliding window, arrays, prefix sumseasy
36. Minimum Absolute DifferencePractice · +10 XP · greedy, sorting, arrayseasy
37. Next ElementPractice · +10 XP · linked structures, arrays, simulationeasy
38. Next Element in ArrayPractice · +10 XP · linked structures, simulation, arrayseasy
39. Parent Array RootPractice · +10 XP · trees, arrayseasy
40. Print Array in ReversePractice · +10 XP · basics, arrayseasy
41. Print Numbers in Reverse OrderPractice · +10 XP · recursion, arrays, iterationeasy
42. Reverse ArrayPractice · +10 XP · recursion, arrays, backtrackingeasy
43. Smallest ElementPractice · +10 XP · greedy, arrays, iterationeasy
44. Stack Push CounterPractice · +10 XP · stack, simulation, arrayseasy
45. Sum of Adjacent ElementsPractice · +10 XP · two pointers, arrays, loopseasy
46. Sum of Array ElementsPractice · +10 XP · recursion, arrays, iterationeasy
47. Sum of Elements in WindowPractice · +10 XP · sliding window, arrayseasy
48. Sum of Next ValuesPractice · +10 XP · linked structures, arrays, simulationeasy
49. The Lucky DipOpen now · Daily · +10 XP · +5 Aura · arrays, max, simple loopeasy
50. Tree Depth SumPractice · +10 XP · trees, arrays, summationeasy
51. Two Pointers SumPractice · +10 XP · two pointers, arrays, two sumeasy
52. Array Prefix Maxima and MinimaPractice · +20 XP · arrays, prefix sums, running max/minmedium
53. Consecutive DuplicatesPractice · +20 XP · arrays, counting, run-length encodingmedium
54. Count Elements with Equal NeighborsPractice · +20 XP · hash map, counting, arraysmedium
55. Count Matching PairsPractice · +20 XP · hash map, counting, arraysmedium
56. Count Pairs with Equal Digit GCDPractice · +20 XP · math, number theory, arraysmedium
57. Count Pairs with Equal Digit SumPractice · +20 XP · math, hashing, arraysmedium
58. Count Pairs with Equal ProductPractice · +20 XP · arrays, hash map, countingmedium
59. Count Pairs with Sum Divisible by KPractice · +20 XP · arrays, hashing, number theorymedium
60. Distinct Consecutive PairsPractice · +20 XP · arrays, hash map, countingmedium
61. Distinct Elements in a Sliding WindowPractice · +20 XP · arrays, hash map, sliding windowmedium
62. Distinct Elements in PrefixPractice · +20 XP · arrays, hash set, prefix sumsmedium
63. Find Closest Smaller ElementPractice · +20 XP · stack, monotonic stack, arraymedium
64. Find Duplicate Node in PathPractice · +20 XP · linked structures, arrays, cyclesmedium
65. Find the First Unique ElementPractice · +20 XP · hash map, arrays, frequencymedium
66. Group by ValuePractice · +20 XP · hash map, grouping, arraysmedium
67. Indices of Next Smaller ElementPractice · +20 XP · stack, monotonic stack, arraymedium
68. Longest Sorted SubsegmentPractice · +20 XP · two pointers, arrays, sliding windowmedium
69. Longest Subarray with At Most K Duplicate PairsPractice · +20 XP · sliding window, hash map, arraysmedium
70. Longest Subarray with Constant DifferencePractice · +20 XP · two pointers, greedy, arraysmedium
71. Max Difference Between Two Prefix SumsPractice · +20 XP · prefix sums, range queries, arraysmedium
72. Maximum Average SubsegmentPractice · +20 XP · binary search, prefix sums, arraysmedium
73. Maximum Prefix Sum DifferencePractice · +20 XP · prefix sums, array, maximummedium
74. Maximum Sum of Consecutive SubsegmentsPractice · +20 XP · dynamic programming, array, greedymedium
75. Maximum Sum of Non-Adjacent Elements in a Circular ArrayPractice · +20 XP · dynamic programming, greedy, arraysmedium
76. Maximum Sum of Non-Adjacent SubarraysPractice · +20 XP · dynamic programming, arrays, greedymedium
77. Minimum Operations to Make Array SortedPractice · +20 XP · greedy, sorting, arraysmedium
78. Next Greater Element on RightPractice · +20 XP · stack, monotonic stack, arraysmedium
79. Stack Maximum DifferencePractice · +20 XP · stack, monotonic stack, arraymedium
80. Subarray Digit SumsPractice · +20 XP · math, prefix sums, arraysmedium
81. Subarray with Target ProductPractice · +20 XP · arrays, hash map, prefix productmedium
82. Sum of Distinct ElementsPractice · +20 XP · recursion, hashing, arraysmedium
83. Sum of Next Greater Element PairsPractice · +20 XP · stack, array, monotonic stackmedium
84. Largest Rectangle in a HistogramPractice · +30 XP · monotonic stack, arrayshard
85. Longest Balanced SubarrayPractice · +30 XP · arrays, hash maps, prefix sumshard
86. Maximum Sum of Non-Adjacent ElementsPractice · +30 XP · basics, dynamic programming, arrayshard
87. Maximum Sum Subarray with Length ConstraintPractice · +30 XP · dynamic programming, arrays, optimizationhard
88. Subarray Minimums with SumPractice · +30 XP · arrays, monotonic stack, prefix sumshard