What is Time Complexity? How is it different from Space Complexity?

0-2 years experience

1. 1Suppose you need to reverse a singly linked list. How would you reason about its time and space complexity?
2. 2If you implement a function that checks whether a string is a palindrome by copying the string first, what impact does that have on time and space compared to an in‑place check?

2-5 years experience

1. 1Your team added a caching layer to a search service, but latency increased unexpectedly. Walk me through how you would use time vs space complexity analysis to diagnose the issue.
2. 2When refactoring a sorting routine from bubble sort to quicksort, what trade‑offs in time and space should you consider for typical input sizes we see in our product?

5-8 years experience

1. 1We need to process a daily log of 500 GB using a map‑reduce job. How would you decide between an algorithm that is O(n log n) time but uses O(1) extra space versus one that is O(n) time but requires O(n) additional memory? Discuss impact on cluster resources and cost.
2. 2Our recommendation engine stores user vectors in memory for fast lookup, but memory pressure is growing. How would you evaluate moving to a more time‑expensive on‑disk representation? Explain the time vs space trade‑offs at scale.

8+ years experience

1. 1The company is migrating a legacy analytics pipeline to a cloud data warehouse. The current implementation uses in‑memory aggregation with O(n) space to achieve linear time. At scale, this is hitting memory limits. How would you lead a cross‑team redesign that balances time and space, considering future data growth and operational costs?
2. 2Across multiple services, we have inconsistent guidelines for algorithmic choices, leading to some teams favoring low‑time but high‑memory solutions and others the opposite. As a staff engineer, how would you establish a policy or framework to evaluate time vs space trade‑offs for new features, and what metrics would you track?