They just wanted the explanation, which felt like a relief at first.
Start by clarifying the problem (e.g., contiguous subarray, return indices or values) and then present the optimal prefix sum with hash map approach. Explain how storing cumulative sums and checking for repeats identifies a zero-sum subarray in O(n) time.
Pro tip: Mention edge cases like empty array, all zeros, and single element zero, and discuss how the solution handles them. Also, briefly compare with the brute-force O(n^2) approach to show awareness of trade-offs.
Ask if the subarray must be contiguous, whether to return indices or the subarray itself, and if there are any constraints on time/space.
Mention that checking all subarrays takes O(n^2) time, which is inefficient for large inputs.
Explain the prefix sum technique: compute cumulative sums and use a hash map to store the first occurrence of each sum. If a sum repeats, the subarray between the two occurrences sums to zero.
Use a small array (e.g., [1, 2, -3, 4]) to illustrate how the hash map detects a zero-sum subarray.
State that time and space are O(n), and discuss handling empty array, no solution, and multiple solutions.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.