Classic problem but I still fumbled the first few minutes trying to remember whether to use the two-pointer approach or precompute prefix/suffix max arrays.
Start by clarifying the problem and walking through a small example to ensure understanding. Then present a solution using precomputed max heights from left and right, explaining how water at each index is determined by the minimum of these maxima minus the bar height. Finally, discuss time and space complexity and possible optimizations.
Pro tip: Mention the two-pointer approach as an O(1) space optimization, but only after presenting the straightforward O(n) space solution. This shows you can balance clarity and efficiency, which Amazon values.
Ask clarifying questions about input constraints, edge cases (e.g., empty array, negative heights), and expected output format. Confirm that water can be trapped only between bars, not outside.
Choose a small array like [0,1,0,2,1,0,1,3,2,1,2,1] and manually compute trapped water to demonstrate understanding and validate your approach.
Describe how to precompute left_max and right_max arrays, then iterate to sum min(left_max[i], right_max[i]) - height[i] for each index. Emphasize that water at each position is limited by the shorter of the two tallest bars on either side.
State that the precomputation approach takes O(n) time and O(n) space. Mention that the two-pointer technique can reduce space to O(1) while maintaining O(n) time.
Cover edge cases like empty array, single bar, or strictly increasing/decreasing heights. Optionally, outline the two-pointer algorithm to show depth.
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