StringPermutation.kt

fun main(args : Array<String>) {
  println("abc".generatePermutations())
}

fun StringBuilder.generatePermutations(start: Int = 0, finish: Int = length-1,
                                       result: MutableSet<String> = HashSet()) : Set<String> {
    if(start == finish) 
        result.add(String(this))
    else
        for(i in start .. finish) {
            swap(start, i)
            generatePermutations(start+1, finish, result)
            swap(start, i)
        }

    return result
}

fun StringBuilder.swap(index1: Int, index2: Int) {

    if(index1 == index2 || index1 < 0 || index2 < 0 || index1 >= length || index2 >= length)
        return
    val tmp = this[index1]
    this[index1] = this[index2]
    this[index2] = tmp
}