Context

In coding interviews, writing a correct solution is only part of the evaluation. Interviewers also expect you to explain its efficiency clearly and justify why it meets the problem constraints.

Core Question

Suppose you implemented a solution that scans a list of Greenhouse Recruiting candidate IDs and uses a hash map to detect whether any ID appears more than once. Explain the time complexity of your solution.

Address these points:

1. What operations dominate the runtime?
2. Why is the overall runtime expressed in Big-O notation as it is?
3. What assumptions are you making about hash map lookups and inserts?
4. What is the corresponding space complexity?

Scope Guidance

You do not need to derive formal proofs, but you should give a precise interview-level explanation. A strong answer should distinguish average-case vs worst-case behavior and connect the complexity to the number of input elements.