Trapping Rain Water

Given a list of non-negative integers representing elevations of columns, and assuming each column is of an equal width of 1, find how much water is trapped in the columns after a spell of rain. The left and right boundaries outside of the columns have 0 elevations.

Input: [3, 2, 1, 2, 2, 3, 2] Output: 5

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1  >>> a = [1, 2, 3]
2  >>> a[-1]
3  3

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