Frequency Maps
Prefix Maps
Composite Keys
Nested Maps
Sliding Window + Map
Grouping / Bucketing
Heap + Map
Caching
Graph + Map
Counting
Difference Maps
1. composite keys
- tuple
(a, b)
- string encoding
3. nested maps
- hierarchical relationships
2. prefix maps
4. hashmap + another DS
- heap → ranking
- linked list → caching
- set → fast membership
use hashmap when you see:
- indexing arbitrary relationships
- trading memory for speed
- turning O(n²) → O(n)
- “find in O(1)”
- “count frequency”
- “pair lookup”
- “prefix reuse”