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

Given a string s, determine if it is valid.
A string s is valid if, starting with an empty string t = "", you can transform t into s after performing the following operation any number of times:

Insert string "abc" into any position in t. More formally, t becomes tleft + "abc" + tright, where t == tleft + tright. Note that tleft and tright may be empty.

Return true if s is a valid string, otherwise, return false.
 
Please complete the following python code precisely:
```python
class Solution:
    def isValid(self, s: str) -> bool:
```