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

You are given a 0-indexed 2D integer array peaks where peaks[i] = [xi, yi] states that mountain i has a peak at coordinates (xi, yi). A mountain can be described as a right-angled isosceles triangle, with its base along the x-axis and a right angle at its peak. More formally, the gradients of ascending and descending the mountain are 1 and -1 respectively.
A mountain is considered visible if its peak does not lie within another mountain (including the border of other mountains).
Return the number of visible mountains.
 
Please complete the following python code precisely:
```python
class Solution:
    def visibleMountains(self, peaks: List[List[int]]) -> int:
```