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.
Both m and n are less than 110. The height of each unit cell is greater than 0 and is less than 20,000.
Given the following 3x6 height map:
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.