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

Given an array of positive integers arr (not necessarily distinct), return the lexicographically largest permutation that is smaller than arr, that can be made with exactly one swap. If it cannot be done, then return the same array.
Note that a swap exchanges the positions of two numbers arr[i] and arr[j]
 
Please complete the following python code precisely:
```python
class Solution:
    def prevPermOpt1(self, arr: List[int]) -> List[int]:
```