Implement a Hash Map from Scratch

Data Structures
Hard
Airbnb
59.5K views

Implement a simple HashMap (without built-in libraries) using an array and linked lists for collision handling (Chaining). Define the Hash function.

Why Interviewers Ask This

Interviewers at Airbnb ask this to evaluate your ability to translate abstract data structure concepts into robust, production-ready code. They specifically test your understanding of memory management, collision resolution strategies like chaining, and hash function design. This question reveals whether you can handle edge cases such as resizing and load factors, which are critical for maintaining the high availability and performance standards required in their distributed travel systems.

How to Answer This Question

1. Clarify Requirements: Immediately confirm if the map needs to support resizing or just basic operations. Mention handling null keys if applicable. 2. Define the Core Structure: Propose a fixed-size array where each index holds a linked list head node to manage collisions via chaining. 3. Design the Hash Function: Explain your choice of a simple modulo operation or a more robust mixing function, emphasizing how it distributes keys uniformly to minimize collisions. 4. Implement Operations: Walk through 'put', 'get', and 'remove'. For 'put', check for existing keys to update values; for 'get', traverse the specific bucket's list. 5. Discuss Edge Cases & Optimization: Address what happens when the load factor exceeds a threshold (e.g., 0.75) and explain the rehashing process to resize the array and redistribute elements. 6. Analyze Complexity: Conclude with time complexity analysis, noting O(1) average case and O(n) worst-case scenarios depending on collision frequency.

Key Points to Cover

  • Explicitly define the hash function logic and why it minimizes collisions
  • Demonstrate clear traversal logic for linked lists during collision handling
  • Address the load factor concept and the necessity of dynamic resizing
  • Correctly handle edge cases like duplicate keys and null values
  • Provide accurate time and space complexity analysis for all operations

Sample Answer

To implement a HashMap from scratch, I would start by defining a Node class for our chaining mechanism, containing key, value, and a next pointer. The core structure will be an array of these nodes. First, I need a hash…

Common Mistakes to Avoid

  • Ignoring the load factor and failing to discuss array resizing, leading to degraded performance
  • Using built-in hash functions without explaining the underlying mechanics or potential pitfalls
  • Forgetting to handle the scenario where a key already exists in the bucket during insertion
  • Implementing open addressing instead of chaining when the prompt specifically requests linked lists

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