What is the average and worst-case time complexity for lookup in a Hash Table?

0-2 years experience

1. 1If you need to store user IDs in a hash map and retrieve them by key, what average time would you expect for a lookup, and what could cause it to degrade to the worst case?
2. 2Suppose you have a hash table with 1000 buckets and you insert 10,000 items. How would you explain the lookup performance for a typical key?

2-5 years experience

1. 1We noticed a spike in latency for a service that uses a hash map for session lookup. Walk me through how hash collisions could lead to worst‑case O(n) lookups and how you would debug it.
2. 2Your team is deciding between a hash table and a balanced BST for a feature that requires frequent key lookups. Explain the trade‑offs in average and worst‑case lookup time and when each structure is preferable.

5-8 years experience

1. 1Design a high‑throughput caching layer that relies on hash tables. How would you ensure that the average O(1) lookup holds under heavy load, and what strategies would you employ to mitigate worst‑case scenarios?
2. 2In a distributed hash table used across multiple nodes, how does load imbalance affect lookup complexity, and what techniques (e.g., consistent hashing, rehashing) can keep performance predictable?

8+ years experience

1. 1Our legacy system uses a custom hash table with a poor hash function, leading to occasional O(n) lookups that impact SLA. As a staff engineer, outline a migration plan to replace it with a more robust solution while minimizing downtime.
2. 2When scaling a global key‑value store, how do you balance hash table design choices (bucket size, resizing policy) against operational concerns like memory fragmentation and GC pauses to maintain average O(1) lookups?