absolute permutation

func absolutePermutation(n: Int, k: Int) -> String {
    let r = (1...n)

    var tmp = Set<Int>()

    var res = ""

    for i in r {
        var a = i + k
        var b = i - k

        if a > b {
            swap(&a, &b)
        }

        let c: Int

        if r.contains(a) && !tmp.contains(a) {
            c = a
        } else if r.contains(b) && !tmp.contains(b) {
            c = b
        } else {
            return "-1"
        }

        tmp.insert(c)

        res.append(String(c))

        if i != n {
            res.append(" ")
        }
    }

    return res
}