Questions
14 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
14 / 17

Explain Separate Chaining. (Buckets/Linked Lists)

Separate Chaining

javascript
  1. 1

    Multiple keys can occupy the same bucket.

  2. 2

    Collision entries are linked or otherwise grouped together.

  3. 3

    Average lookup remains close to O(1) when the load factor is controlled.

  4. 4

    Worst-case lookup can become O(n) if many keys fall into one bucket.

  5. 5

    Separate chaining makes deletion simpler than many open-addressing designs.

  6. 6

    Modern implementations may treeify heavily populated buckets.

Difficulty: 5/10
Topics: collision resolution, hash tables, linked lists

Scenario Questions

0-2 years experience
  1. 1

    If you need to implement a hash map for storing usernames, how would you handle collisions using separate chaining?

  2. 2

    What happens when you insert a key that hashes to a bucket that already contains a linked list of entries? Walk me through the steps.

2-5 years experience
  1. 1

    We have a service that uses a hash table with separate chaining, and after a traffic spike we notice lookups are slower. What could be causing the slowdown and how would you investigate?

  2. 2

    Suppose we need to support delete operations in a hash table that uses separate chaining. How would you implement delete, and what edge cases must you watch out for?

5-8 years experience
  1. 1

    Design a thread‑safe hash map for a high‑throughput caching layer that uses separate chaining. What synchronization strategy would you choose and why?

  2. 2

    If the average chain length grows to 10 because the load factor is high, what redesign options do you consider to keep performance predictable at scale?

8+ years experience
  1. 1

    Our legacy system uses a custom hash table with separate chaining and is being migrated to a distributed key‑value store. How would you plan the migration to minimize downtime and ensure data consistency?

  2. 2

    Across multiple services, some teams prefer open addressing while others use separate chaining. How would you evaluate the long‑term maintainability and performance trade‑offs to set a company‑wide standard?

Follow-up Questions

  • ❓Can you compare the memory overhead of separate chaining versus open addressing?
  • ❓How does the choice of linked‑list implementation affect cache locality?
  • ❓What would you do if the hash function degrades and many keys collide?