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

Given a positive integer num, return true if num is a perfect square or false otherwise.
A perfect square is an integer that is the square of an integer. In other words, it is the product of some integer with itself.
You must not use any built-in library function, such as sqrt.
 
Please complete the following python code precisely:
```python
class Solution:
    def isPerfectSquare(self, num: int) -> bool:
```