public class Solution
extends Object
407 - Trapping Rain Water II\.
Hard
Given an `m x n` integer matrix `heightMap` representing the height of each unit cell in a 2D elevation map, return _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.length`
* `n == heightMap[i].length`
* `1 <= m, n <= 200`
* 0 <= heightMap[i][j] <= 2 * 104