Find Maximum Area of a Triangle

Given n points, return twice the maximum area of any triangle whose at least one side is axis-parallel (or -1 if none); this reduces to maximizing base*height where the base is the distance between two points sharing x or y and the height is the perpendicular distance to a third point. Key idea: consider pairs of points with equal x or equal y and combine with a third point at a different coordinate to maximize base*height (n up to 1e5 so group/scan efficiently).