AlgoDaily - Trapping Rain Water

Code for Trapping Rain Water

This code is intentionally made very precise and concise by following Functional Programming principles. We do the following step by step to get the result:

First we check the corner/simplest cases and return 0 if found.
So our final result will be a sum of something.
sum of what? sum of minimum of two values calculated from heights. We also need to subtract heights[i] from them.
min of which two values? Values calculated from left-to-right at each index and right-to-left at each index.
So we can return all these with that one statement.

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def totalRain(heights):
    if not heights or len(heights) <= 2:
        return 0
    
    return sum(
        [min(
            max(heights[:i+1] or heights[:1]),
            max(heights[i:] or heights[-1:])
            ) - heights[i] for i in range(len(heights))
        ]
    )
​
​
if __name__ == '__main__':
        # uncomment a different array to see the working on a different array
        # myArr = [0, 0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1, 0, 0] # should be 6
        # myArr = [0, 0, 1, 0, 2, 4, 4, 4, 2, 3, 1, 0, 2, 4, 3, 1, 0, 1] # should be 14
        # myArr = [0, 0, 1, 2, 4, 4, 4, 3, 1, 0, 0, 0] # should be 0
        myArr = [5, 1, 2, 4, 4, 4, 3, 1, 0, 0, 0] # should be 5
​
        print(totalRain(myArr))

OUTPUT

Results will appear here.