Water collected between towers

Question

I recently came across an interview question asked by Amazon and I am not able to find an optimized algorithm to solve this question:

You are given an input array wh

Answer 1

Here's an efficient solution in Haskell

rainfall :: [Int] -> Int
rainfall xs = sum (zipWith (-) mins xs)
    where mins = zipWith min maxl maxr
          maxl = scanl1 max xs
          maxr = scanr1 max xs

it uses the same two-pass scan algorithm mentioned in the other answers.