Please solve the programming task below using a self-contained code snippet in a markdown code block.

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.
 
Please complete the following python code precisely:
```python
class Solution:
    def trapRainWater(self, heightMap: List[List[int]]) -> int:
```