Trapping Rain Water II
Hard
Provided an m x n integer matrix heightMap representing the height of each unit cell in a 2D elevation map, produce the volume of water it can trap after raining.
Example 1
Input: heightMap = [[1,4,3,1,3,2],[3,2,1,3,2,4],[2,3,3,2,3,1]]
Output: 4
Explanation: After the rain, water is trapped between the blocks.
We have two small ponds 1 and 3 units trapped.
The total volume of water trapped is 4.
Example 2
Input: heightMap = [[3,3,3,3,3],[3,2,2,2,3],[3,2,1,2,3],[3,2,2,2,3],[3,3,3,3,3]]
Output: 10
Constraints
m == heightMap.lengthn == heightMap[i].length1 <= m, n <= 2000 <= heightMap[i][j] <= 2 * 10^4
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