what about crash?

Sadly, I am still using Xcode 6.3.1 for ineffable production reasons, however this more elegant solution worked for me. I’d appreciate your feedback if it could be better (including safer).

let a = [[1, 2, 3, [4, 5, [6,7,8]]
let b = [[1, 2, 3, [4, [5,9], [6,7,8]]
let c = [1, 2, 3, 4, 5]
import Foundation
func NSFlatten(inputArray : [NSObject]) -> [NSObject] {
    var result = [NSObject]()
    for item in inputArray {
        if let array = item as? [NSObject] {
                result += NSFlatten(array)
        } 
        else {
                result.append(item)
        }
    }
    return result
}
NSFlatten(a)  // [1, 2, 3, 4, 5, 6, 7, 8]
NSFlatten(b)  // [1, 2, 3, 4, 5, 9, 6, 7, 8]
NSFlatten(c)  // [1, 2, 3, 4, 5]