trapping water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
Examples:

Input: arr[]   = {2, 0, 2}
Output: 2
Structure is like below
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We can trap 2 units of water in the middle gap.

Input: arr[]   = {3, 0, 0, 2, 0, 4}
Output: 10
Structure is like below
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We can trap "3*2 units" of water between 3 an 2,
"1 unit" on top of bar 2 and "3 units" between 2 
and 4.  See below diagram also.

Variation: if elevation of 0 means there is wall between it and no water can be trapped between those walls. What will be the changes to your solution?

Variation sample: => only 2 units of water now since there is gap of 0 in the middle

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