struct Rectangle { let length: Double? init(frombreadth breadth: Double) { length = breadth * 10 } init(frombre bre: Double) { length = bre * 30 } init(_ area: Double) { length = area } } let rectarea = Rectangle(180.0) print("面积为：\(rectarea.length)") let rearea = Rectangle(370.0) print("面积为：\(rearea.length)") let recarea = Rectangle(110.0) print("面积为：\(recarea.length)")