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All leaked interview problems are collected from Internet.

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.

**Note:**

Both *m* and *n* are less than 110. The height of each unit cell is greater than 0 and is less than 20,000.

**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 are trapped between the blocks. The total volume of water trapped is 4.

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