A scalable way to remove duplicates from Arrays without extensions in Swift 4

Question

This method is an attempt at removing duplicates from arrays without the use of extensions.

func arrayWithDuplicatesRemoved<T: Equatable>(from array: [T]) -> [T] {
    var results = [T]()
    return array.compactMap { (element) -> T? in
        if results.contains(element) {
            return nil
        } else {
            results.append(element)
            return element
        }
    }
}

let dirty = ["apple", "kiwi", "grapefruit", "kiwi", "kiwi", "strawberry", "watermelon", "apple", "banana"]
let clean = arrayWithDuplicatesRemoved(from: dirty)
print(clean) // ["apple", "kiwi", "grapes", "strawberry", "watermelon", "banana"]

Answer 1

Your algorithm is O(n^2) so not very scalable... The immediate improvement I can think of is to use a Set for results (which means making T Hashable).

If you aren't worried about position, then just create a Set from an array and you are done.

Answer 2

UPDATE

The original was not the most scalable; nesting how I did meant that the square of the size of the argument is how performance is measured (thanks to Daniel T. for pointing that out). This is much more scalable as performance is not entirely linear but much closer than before.

func ArrayWithDuplicatesRemovedFrom<T: Hashable>(array: [T]) -> [T] {
    return Array(Set(array))
}