Swift: second occurrence with indexOf

Question

let numbers = [1,3,4,5,5,9,0,1]

To find the first 5, use:

numbers.indexOf(5)

How do I find the secon

Answer 1

extension Collection where Element: Equatable {
    func nth(occurance: Int, of element: Element) -> Index? {
        var level : Int = occurance
        var position = self.startIndex
        while let index = self[position...].index(of: element) {
            level -= 1
            guard level >= 0 else { return nil }
            guard level != 0 else { return index }
            position = self.index(after: index)
        }
        return nil
    }
}