Before revision:  This problem was introduced by Funke, Krumpe, and Storandt IWOCA 2016 . We provide the first general subquadratic algorithm for computing the times and the order of disappearance. This algorithm also works for other shapes ( such as rectangles)and in any fixed dimension.
After revision:  This problem was introduced by Funke, Krumpe, and Storandt IWOCA 2016 . We provide the first general subquadratic algorithm for this problem. Our solution extends to other shapes ( such as rectangles)and in any fixed dimension.