The other answers are all correct, but don't forget you could also use collection operators, as follows:
var list = [1, 2, 3, 4]
var max: Int = (list as AnyObject).valueForKeyPath("@max.self") as Int
you can also find the average in the same way:
var avg: Double = (list as AnyObject).valueForKeyPath("@avg.self") as Double
This syntax might be less clear than some of the other solutions, but it's interesting to see that -valueForKeyPath:
can still be used :)