let dictionary: [Int: String?] = [1:"one", 2:nil, 3:"three"]

// It can be taken out as LazyMapCollection. *The order is irregular.
let underTwoInt = [Int](dictionary.keys.filter { $0 <= 2 }).sort()
print(underTwoInt) // [1, 2]

let nonNullString = [String](dictionary.values.flatMap { $0 }).sort()
print(nonNullString) // ["one", "three"]