Can you implement a HashMap from scratch in any language?
This task requires designing a hash map data structure, handling collisions, and implementing basic operations like put, get, and remove.
Why Interviewers Ask This
Implementing a HashMap tests deep understanding of hashing functions, collision resolution strategies (chaining or open addressing), and dynamic resizing. It reveals how well you can translate abstract data structure concepts into concrete code. Interviewers look for your ability to handle edge cases, manage memory efficiently, and explain trade-offs between different collision handling methods.
How to Answer This Question
Define a class structure with an internal array (buckets) and a size variable. Implement a hash function to map keys to indices. Discuss collision resolution: choose chaining (linked lists) or open addressing. Write the put method to insert key-value pairs, handling collisions. Implement get to retrieve values and remove to delete entries. Mention rehashing when load factor exceeds a threshold to maintain performance.
Key Points to Cover
- Hash function implementation
- Collision resolution strategy selection
- Dynamic resizing logic
- Time complexity considerations
Sample Answer
I would create a HashMap using an array of buckets. The hash function computes an index for each key. For collisions, I'll use chaining, storing linked lists at each bucket. The put method calculates the hash, finds the bucket, and appends the entry. The get method traverses the list at the calculated index to find the key. When the number of elements grows too large, I resize the array and rehash all existing entries to maintain O(1) average time complexity.
Common Mistakes to Avoid
- Ignoring null keys or values
- Not handling collisions correctly
- Forgetting to rehash during resizing
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