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]
3