Recognition signal
When should you think of Arrays & Hashing?
You need constant-time lookup, frequency counts, deduplication, or information about a prefix.
28 worked problems
Arrays & Hashing
Frequency maps, prefix sums, in-place scans. You need constant-time lookup, frequency counts, deduplication, or information about a prefix.
Core invariant
What must stay true?
The map or prefix state summarizes exactly the portion already scanned.
Practice in a deliberate order
Starter
Build the template
Core
Recognize variations
Stretch
Combine ideas under pressure
What to learn in this cluster
- 01Direct hash lookup
- 02Frequency maps and grouping
- 03Prefix sums and counting subarrays
All Arrays & Hashing problems
28 of 28 walkthroughsNo matching problems. Try another title, tag, or difficulty.
- #1 Two Sumarray · hash-tableEasy
- #13 Roman to Integerstring · hash-table · mathEasy
- #118 Pascal's Trianglearray · dpEasy
- #121 Best Time to Buy and Sell Stockarray · dp · greedyEasy
- #228 Summary RangesarraysEasy
- #485 Max Consecutive OnesarrayEasy
- #706 Design HashMaphash-table · design · linked-listEasy
- GFG Leaders in an Arrayarray · suffixEasy
- #18 4Sumarray · two-pointers · sortingMedium
- #50 Pow(x, n)math · recursion · divide and conquerMedium
- #53 Maximum Subarrayarray · dp · kadaneMedium
- #128 Longest Consecutive Sequencearray · hash-set · union-findMedium
- #151 Reverse Words in a Stringstring · two-pointersMedium
- #169 Majority Elementarray · boyer-moore · countingMedium
- #189 Rotate Arrayarray · two-pointers · in-placeMedium
- #229 Majority Element IIarray · boyer-moore · countingMedium
- #238 Product of Array Except Selfarray · prefix-product · suffix-productMedium
- #287 Find the Duplicate Numberarray · two-pointers · floydMedium
- #380 Insert Delete GetRandom O(1)array · hash-table · designMedium
- #2149 Rearrange Array Elements by Signarray · two-pointersMedium
- GFG Longest Subarray with Sum Karray · prefix-sum · hash-tableMedium
- #41 First Missing Positivearray · in-place-hashing · cyclic-sortHard
- #149 Max Points on a Linearray · hash-table · geometryHard
- #315 Count of Smaller Numbers After Selfarray · merge-sort · divide-and-conquerHard
- #493 Reverse Pairsarray · merge-sort · divide-and-conquerHard
- GFG Count Subarrays with XOR Karray · bit-manipulation · hash-tableHard
- GFG Find Missing and Repeating Numberarray · math · bit-manipulationHard
- GFG Count Inversionsarray · merge-sort · divide-and-conquerHard
Continue the cluster