Hash Map vs Hash Set
Learn when to use each pattern and make the right choice in your coding interview.
Quick Answer
Use Hash Map when you need to store key-value pairs or count frequencies
Use Hash Set when you only need to track presence/absence
Side-by-Side Comparison
Hash Map
Best For
Two Sum, frequency counting, caching
Time
O(1) average
Space
O(n)
Hash Set
Best For
Contains Duplicate, finding unique elements
Time
O(1) average
Space
O(n)
How to Decide
- Need to store additional data with each key? → Hash Map
- Just checking "have I seen this before"? → Hash Set
- Counting occurrences? → Hash Map
- Removing duplicates? → Hash Set
Practice Both Patterns
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