Started with brute force, check every subarray and verify if sorting it fixes the whole thing.
Start by clarifying the problem and edge cases, then present a brute-force solution and optimize it. Discuss two main approaches: sorting a copy and comparing (O(n log n)) and a two-pointer linear scan (O(n)). Compare their time/space tradeoffs and choose the linear approach for efficiency.
Pro tip: Emphasize that the linear approach is optimal and explain why it works by identifying the first and last elements that violate the non-decreasing order. Mention that you would test with edge cases like already sorted arrays and arrays with duplicates.
Restate the problem in your own words and ask clarifying questions about input constraints, expected output, and edge cases (e.g., empty array, single element, already sorted).
Propose a simple solution: create a sorted copy of the array, compare with the original to find the first and last mismatched indices, and return the length. Analyze its O(n log n) time and O(n) space complexity.
Explain the two-pointer method: find the leftmost index where the array stops being non-decreasing, and the rightmost index where it stops being non-decreasing from the right. Then expand these boundaries to include any elements that would break the sorted order if the subarray were sorted.
Discuss the tradeoffs: the sorting approach is simpler but uses extra space and is slower; the two-pointer approach is optimal in time and space but requires careful implementation to handle edge cases.
Walk through a few examples (e.g., [2,6,4,8,10,9,15], [1,2,3,4], [1]) to demonstrate correctness and edge-case handling.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.