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

Given the string s, return the size of the longest substring containing each vowel an even number of times. That is, 'a', 'e', 'i', 'o', and 'u' must appear an even number of times.
 
Please complete the following python code precisely:
```python
class Solution:
    def findTheLongestSubstring(self, s: str) -> int:
```