Question presented to candidate: "Before we start the coding part: walk me through how you approach a data-structures problem you have not seen before. Then show me on a small problem — count the pairs in a list that add up to a target — and write it in Python."
What a strong answer should cover:
- A clear, repeatable sequence: clarify, examples, brute force, optimise, code, test — and talking through each step.
- Clarifying questions that change the algorithm (input size, duplicates, negatives, empty input, output format).
- Stating the brute force and its complexity before optimising, then naming the bottleneck and the data structure that removes it.
- The space-time trade-off of the optimised version (O(n) time for O(n) extra memory).
- Testing: a dry run, edge cases, and how a stress test against the brute force would catch bugs.
- Python specifics:
dict.get(key, 0)orcollections.Counter/defaultdict(int)for counting, integers never overflow, and the default recursion limit of 1000 matters for deep recursion.
Clarifying questions expected:
- "How large can the input be, and can values repeat or be negative?"
- "Do you want the number of pairs, or the pairs themselves?"
Code / implementation expected: Yes — a brute force and an O(n) solution in Python using a dict, plus a few test calls.