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What Is Hash Table?
Hash Table Data Structure
A hash table maps keys to values using a hash function that converts keys to array indices. Lookups, insertions, and deletions are O(1) on average. JavaScript Objects and Maps, Python dicts, and Java HashMaps are all hash tables. It's the most practically useful data structure.
How Hash Table Works
Hash function: hash('alice') → 42. Store Alice's data at index 42 of an array. To retrieve: hash('alice') → 42, look at index 42. Constant time regardless of table size. Collisions (two keys mapping to the same index) are handled by chaining (linked lists) or open addressing.
Key Concepts
- Hash Function — Converts a key to an array index — must be deterministic and distribute keys evenly
- O(1) Average — Constant time operations — lookup, insert, delete in average case
- Collision Handling — When two keys hash to the same index — resolved by chaining or probing
- Load Factor — Ratio of entries to table size — tables resize (rehash) when load factor exceeds threshold
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Frequently Asked Questions
When should I use a hash table?
Whenever you need fast key-based lookup — counting occurrences, detecting duplicates, caching, implementing sets. If you're using an array and searching it linearly, consider a hash table.
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