Hash Maps
Key–value lookups, counting, and frequency maps.
Foundation
Learn the invariant, apply it independently, then pass the Hash maps checkpoint.
- Build a frequency table (Easy)
Checkpoint
A graded check of the core Hash maps reasoning skills.
Pattern Quest
Practice recognizing hash maps problem shapes.
Hash maps
Practice turning repeated searches into direct lookup with frequency maps, complement tables, grouping keys, and carefully chosen stored state.
- Find K closest to center (Easy)
- Find Kth largest (Medium)
- Find most popular cost books (Easy)
- Find top K frequent elements (Medium)
- Find top K tweet words in a recent window (Medium)
- Find X to make array sum to K (Medium)
- Find year with most population (Medium)
- Object as key (Easy)
- Remove duplicate contacts (Medium)
- Sort pairs (Medium)