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

Given three integers x, y, and bound, return a list of all the powerful integers that have a value less than or equal to bound.
An integer is powerful if it can be represented as xi + yj for some integers i >= 0 and j >= 0.
You may return the answer in any order. In your answer, each value should occur at most once.
 
Please complete the following python code precisely:
```python
class Solution:
    def powerfulIntegers(self, x: int, y: int, bound: int) -> List[int]:
```