String permutations in a way that makes some sense

Permutations.swift

import Foundation

func permute(_ s: String) -> [String] {
    var arr = Array(s)
    var permutations = [[Character]]()
    permute(&arr, 0, &permutations)
    return permutations.map { String($0) }
}

func permute<T>(_ arr: inout [T], _ k: Int, _ result: inout [[T]]) {
    for i in k ..< arr.count {
        arr.swapAt(i, k)
        permute(&arr, k + 1, &result)
        arr.swapAt(k, i)
    }
    if k == arr.count - 1 { result.append(arr) }
}

print(permute("whoah"))