Given an $m \times n$ integer matrix heightMap, where each cell represents the height of a unit cell in a 2D elevation map, return the total volume of water that can be trapped after raining.

Constraints:

• m $=$ heightMap.length

• n $=$ heightMap[i].length

• $1 \leq$ m, n $\leq 200$

• $0 \leq$ heightMap[i][j] $\leq 2 \times 10^4$

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