Classic problem but I still fumbled the first few minutes trying to think of a two-pointer approach before remembering you just track the minimum price seen so far and update max profit on each step.
Start by clarifying the problem constraints (e.g., single transaction, array size, price types) and then propose a one-pass solution that tracks the minimum price seen so far and the maximum profit. Explain the O(n) time and O(1) space complexity, and walk through a small example to demonstrate correctness.
Pro tip: Mention that you would handle edge cases like empty array or decreasing prices by returning 0, and that you would discuss potential follow-ups (e.g., multiple transactions) to show depth.
Ask about input constraints (array size, price range, whether prices are integers), and confirm that only one buy and one sell are allowed, with buy before sell.
Explain that you will iterate through the array once, keeping track of the minimum price seen so far and the maximum profit achievable.
Use a small array like [7,1,5,3,6,4] to show how the algorithm updates min price and max profit step by step.
State that the time complexity is O(n) and space complexity is O(1), which is optimal for this problem.
Mention that if the array is empty or prices are strictly decreasing, the algorithm returns 0, ensuring no negative profit.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.