← Snowflake Interview Insights
The two-pointer setup clicked for me pretty fast, iterate l and push r as far right as it'll go while the condition holds.
First, restate the problem in your own words and clarify the definitions of the prefix array and the condition. Then, explain that the inequality can be transformed into prefix[r] - k <= 2*prefix[l], which suggests using a data structure to efficiently find the smallest l for each r. Finally, describe an O(n log n) or O(n) algorithm using a monotonic stack or binary search on a decreasing sequence of prefix values.
Pro tip: Mention that the prefix array is non-decreasing if the original array has non-negative numbers, but the algorithm should handle arbitrary integers. Also, emphasize that you would test edge cases like empty subarrays and large k.
Restate the problem and confirm the definition of the prefix array and the condition. Ask if the array can contain negative numbers and if the subarray must be non-empty.
Rewrite the condition prefix[r] - 2*prefix[l] <= k as prefix[r] - k <= 2*prefix[l]. This makes it easier to see that for each r, we need the smallest l such that 2*prefix[l] >= prefix[r] - k.
Propose using a monotonic stack to maintain indices with decreasing 2*prefix values, then for each r, binary search to find the smallest l satisfying the condition. Alternatively, use a two-pointer approach if the array is non-negative.
State that the monotonic stack + binary search approach runs in O(n log n) time and O(n) space. If the array is non-negative, a two-pointer approach can achieve O(n) time.
Walk through a small example to verify the algorithm, and discuss edge cases such as all elements negative, k very large, or no valid subarray.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.