Context

Debugging is a core coding skill in interviews, especially when a correct-looking solution fails on edge cases or only under specific inputs. Interviewers want to see a structured process, not just the final fix.

Core Question

Describe a challenging bug you encountered in algorithmic code and how you resolved it. In your answer, explain:

1. How you identified the symptoms and narrowed the failure to a specific part of the code.
2. What debugging techniques you used such as reproducing the issue, testing edge cases, inspecting state, or simplifying the input.
3. What the root cause was, why it was easy to miss, and how you fixed it.
4. How you verified the fix and prevented similar bugs in the future.

Scope Guidance

Give a concrete example involving a small algorithm or data-structure problem, such as duplicate detection, off-by-one indexing, incorrect hash map updates, or mutation of shared state. The interviewer expects a clear, technical explanation with a beginning, middle, and end, plus lessons learned about debugging discipline.