Answer #3

func getMinimumDifference(a: [String], b: [String]) -> [Int] {
    return zip(a, b).map { (f, n) -> Int in
        guard f.count == n.count else { return -1 }
//      return Set(f).subtracting(Set(n)).count
        return f.count - f.filter({ n.contains($0) }).count
    }
}