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

• cumulative sums / states

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”