Implement HashMap with Separate Chaining
Implement the core logic of a hash map using an array of linked lists (separate chaining) to handle collisions. Focus on the `put`, `get`, and `resize` logic.
Why Interviewers Ask This
Amazon interviewers ask this to evaluate your ability to handle collision resolution and memory management under pressure. They specifically test if you understand how separate chaining works, can implement efficient hash functions, and know when to trigger a resize operation to maintain O(1) performance. This mirrors Amazon's expectation for engineers who build scalable systems that handle edge cases without degrading user experience.
How to Answer This Question
1. Clarify Requirements: Ask about constraints like key types, null handling, and expected load factor thresholds before coding. 2. Define Structure: Propose an array of linked list nodes where each node stores the key, value, and next pointer. 3. Implement Hash Function: Write a simple but robust hash function using modulo arithmetic to map keys to indices. 4. Code Core Methods: First write 'put' to insert or update values, then 'get' to retrieve them, ensuring you traverse the chain correctly. 5. Add Resize Logic: Explain the rehashing process where you double the array size and redistribute all elements to minimize collisions. 6. Analyze Complexity: Explicitly state time complexity for average vs. worst-case scenarios and discuss space trade-offs.
Key Points to Cover
- Explicitly explain the collision resolution strategy using linked lists
- Demonstrate understanding of load factors and dynamic resizing mechanics
- Provide correct hash function implementation with modulo arithmetic
- Analyze both average case O(1) and worst case O(n) complexities
- Handle edge cases like duplicate keys and null inputs cleanly
Sample Answer
I would start by defining a Node class containing the key, value, and a reference to the next node. The HashMap will use an array of these nodes as buckets. For the put operation, I calculate the index using the hash cod…
Common Mistakes to Avoid
- Forgetting to handle the case where a key already exists in the bucket during insertion
- Implementing a hash function that returns negative values without proper modulo adjustment
- Neglecting to mention the load factor threshold that triggers a resize operation
- Failing to rehash all existing elements when doubling the array size
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