How do you implement the Trapping Rain Water problem efficiently?
This problem calculates how much water can be trapped between bars of varying heights after raining. It tests stack usage or precomputed arrays.
Why Interviewers Ask This
Interviewers ask this to test complex problem-solving skills involving boundary conditions and cumulative calculations. They want to see if you can determine the water level at each index based on the maximum heights to its left and right. It demonstrates mastery of multi-pass algorithms or stack-based solutions.
How to Answer This Question
Explain that water at any index depends on the minimum of the max height to its left and right. Propose the precomputation method: create two arrays storing max heights from left and right. Calculate trapped water for each index using these arrays. Alternatively, discuss the two-pointer approach for O(1) space. Walk through the logic of why the smaller boundary determines the water level.
Key Points to Cover
- Boundary dependency logic
- Precomputation of max heights
- Water level calculation
- Space optimization techniques
Sample Answer
To solve this, I first realize that the water level at any bar is determined by the shorter of the tallest bars on either side. I can precompute the maximum height to the left and right of every index. By iterating throu…
Common Mistakes to Avoid
- Ignoring the case where no water is trapped
- Incorrectly updating max height arrays
- Failing to handle edge cases at boundaries
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