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

You may recall that an array arr is a mountain array if and only if:

arr.length >= 3
There exists some index i (0-indexed) with 0 < i < arr.length - 1 such that:
	
arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
arr[i] > arr[i + 1] > ... > arr[arr.length - 1]



Given an integer array arr, return the length of the longest subarray, which is a mountain. Return 0 if there is no mountain subarray.
 
Please complete the following python code precisely:
```python
class Solution:
    def longestMountain(self, arr: List[int]) -> int:
```