Design HashMap

Design HashMap

1. What is this problem about?

The Design HashMap interview question asks you to implement a key-value store from scratch without using any built-in library hash tables. You need to support put(key, value), get(key), and remove(key). This Design HashMap coding problem is a fundamental test of how well you understand the internals of data structures, specifically collision handling and indexing.

2. Why is this asked in interviews?

Tech giants like Apple, Google, and Microsoft ask this to verify your understanding of Hash Function interview patterns. It evaluates whether you know how to map a large range of keys to a smaller array size and how to handle two keys mapping to the same index (collisions). It's a deep dive into computer science fundamentals.

3. Algorithmic pattern used

This problem is typically solved using Separate Chaining.

• Array of Buckets: Create an array of a fixed size (e.g., 1000).
• Hash Function: Use key % size to find the index.
• Collision Resolution: Each element in the array is a Linked List (or a simple list of pairs).
• Operations:
  • put: Find the index, traverse the list to update an existing key, or append a new pair.
  • get: Find the index, traverse the list to find the key.
  • remove: Find the index, traverse and remove the node from the linked list.

4. Example explanation

Bucket size = 10.

1. put(1, 100): $1 \% 10 = 1$. Add (1, 100) to Bucket 1.
2. put(11, 200): $11 \% 10 = 1$. Bucket 1 already has (1, 100). Add (11, 200) to the same bucket (Chaining).
3. get(11): Go to Bucket 1, see 1 is not 11, move to next, found 11. Return 200.
4. remove(1): Go to Bucket 1, find node with key 1, remove it.

5. Common mistakes candidates make

• Huge Array: Creating an array of size $10^6$ to avoid collisions. This uses too much memory and isn't a "true" hash map implementation.
• Ignoring Collisions: Failing to handle the case where multiple keys map to the same bucket.
• Linear Remove: Forgetting that removal in a linked list requires keeping track of the previous node or using a specific list removal method.

6. Interview preparation tip

Learn the difference between Separate Chaining and Open Addressing (like linear probing). Being able to compare their performance under load shows that you understand the practical engineering challenges of high-performance data storage.