https://stackoverflow.com/a/52136303/1187415

permutations.swift

import Foundation

func permute<E>(i: Int, array: [E]) -> [E] {
    var i = i
    var result = array
    for j in 0..<array.count {
        let k = i % (array.count - j)
        i /= (array.count - j)
        result.swapAt(j, j + k)
    }
    return result
}


for i in 0 ..< 1*2*3 {
    print(permute(i: i, array: ["a", "b", "c"]).joined())
}