Questions
10 of 17
1What is the average and worst-case time complexity for lookup in a Hash Table?
2What is a Hash Collision?
3What is the difference between HashMap, TreeMap, and LinkedHashMap?
4Why is the worst-case lookup O(n)? How do modern implementations avoid this? (Tree buckets)
5What are the security implications of Hash Collision attacks (HashDoS)?
6What is the Load Factor? How does it trigger a Rehash/Resize?
7What is a Bloom Filter? When is it used? (Cache penetration, Spell check)
8What is a Hash Table? What is a Key-Value pair?
9What is a Cryptographic Hash? How does it differ from a regular hash?
10How do you design a Hash Table from scratch?
11What is a Hash Function? What makes a good hash function?
12What is the significance of Equals() and GetHashCode() in C#/Java when using objects as keys?
13What is Consistent Hashing? How is it used in Distributed Systems?
14Explain Separate Chaining. (Buckets/Linked Lists)
15Explain Open Addressing. (Linear Probing, Quadratic Probing, Double Hashing)
16Why can't we use a Hash Table efficiently for Ordered operations (e.g., find min/max)?
17Why is it bad to use a mutable object as a key in a HashMap?
Data-StructuresData-Structures
Basics
Complexity Analysis
Arrays and Strings
Linked Lists
Stacks and Queues
Hash Tables
10 / 17

How do you design a Hash Table from scratch?

Hash Table Design

javascript
  1. 1

    Choose a bucket array and entry representation.

  2. 2

    Implement deterministic hashing and bucket-index calculation.

  3. 3

    Use separate chaining or open addressing for collisions.

  4. 4

    Implement get, put, contains, and remove.

  5. 5

    Use a load-factor threshold to trigger resizing.

  6. 6

    Rehash all entries after resizing.

  7. 7

    Ensure key equality and hashing are consistent.

  8. 8

    Define null-key behavior and duplicate-key semantics.

  9. 9

    Consider iteration, memory overhead, concurrency, and security requirements.

  10. 10

    Test collision-heavy, resize-heavy, empty, duplicate, and adversarial cases.

Difficulty: 4/10

Follow-up Questions

  • ❓How would you implement resizing?
  • ❓How would you handle deletion with open addressing?
  • ❓How would you make the Hash Table thread-safe?