Hashing & Frequency Maps Pattern
Hash maps and sets turn membership, counting, and grouping into constant-time operations. They're the most common single tool in interview problems — often the difference between O(n²) and O(n).
Typical complexity: O(n) time, O(n) space (amortized O(1) per op)
When to use it
- You need fast lookups, counts, or dedupe.
- You're grouping items by a key (anagrams, buckets).
- A nested loop is just checking 'have I seen this before?'.
Practice problems
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