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Given an m x n
matrix of positive integers representing the height of each unit cell in a 2D elevation map, compute the volume of water it is able to trap after raining.
Example:
Given the following 3x6 height map:
[
[1,4,3,1,3,2],
[3,2,1,3,2,4],
[2,3,3,2,3,1]
]
Return 4.

The above image represents the elevation map [[1,4,3,1,3,2],[3,2,1,3,2,4],[2,3,3,2,3,1]]
before the rain.

After the rain, water is trapped between the blocks. The total volume of water trapped is 4.
Constraints:
1 <= m, n <= 110
0 <= heightMap[i][j] <= 20000
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