Can you describe how to implement a HashMap from scratch?
This question asks for a deep dive into hash table mechanics, including collision handling and resizing strategies.
Why Interviewers Ask This
HashMaps are fundamental data structures used everywhere in software development. Implementing one from scratch shows a deep understanding of hashing, collision resolution (chaining or open addressing), and dynamic resizing. It reveals whether the candidate understands the trade-offs between time complexity and space usage.
How to Answer This Question
Explain the core concept of using a hash function to map keys to indices. Discuss collision handling methods like chaining (linked lists) or open addressing. Explain the need for rehashing when the load factor exceeds a threshold. Mention time complexities for insert, delete, and lookup operations under average and worst-case scenarios.
Key Points to Cover
- Hash function role
- Collision resolution techniques
- Load factor and resizing
- Time complexity analysis
Sample Answer
A HashMap stores key-value pairs using a hash function to compute an index into an array of buckets. When collisions occur, I would use chaining, linking entries with the same index into a list. As the number of elements…
Common Mistakes to Avoid
- Ignoring collision scenarios
- Forgetting to explain resizing
- Overlooking worst-case performance
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