Two Pointers moving in opposite directions

Trapping Rain Water

Given a list of non-negative integers representing elevations of columns, and assuming each column is of equal width of 1, find how much water is trapped in the columns after a rain. 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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  >>> a = [1, 2, 3]
  >>> a[-1]

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