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

An additive number is a string whose digits can form an additive sequence.
A valid additive sequence should contain at least three numbers. Except for the first two numbers, each subsequent number in the sequence must be the sum of the preceding two.
Given a string containing only digits, return true if it is an additive number or false otherwise.
Note: Numbers in the additive sequence cannot have leading zeros, so sequence 1, 2, 03 or 1, 02, 3 is invalid.
 
Please complete the following python code precisely:
```python
class Solution:
    def isAdditiveNumber(self, num: str) -> bool:
```