Beautiful implementation of quick sort on Kotlin

QuickSort.kt

package quicksort

import java.util.Collections


fun main(args : Array<String>) {
    val list = "abcdefghijklmnopqrstuvwxyz".toMutableList()
    Collections.shuffle(list)
    println("before sort: $list")
    list.quicksort()
    println("after sort: $list")
}



fun <T> MutableList<T>.swap(index1: Int, index2: Int) {
    val tmp = this[index1]
    this[index1] = this[index2]
    this[index2] = tmp
}


fun <T: Comparable<T>> MutableList<T>.quicksort(low: Int = 0, high: Int = this.size - 1) {
    if(this.isEmpty())
        return

    val pivot = this[low+(high - low)/2]
    var i = low
    var j = high

    while(i <= j){
      
        while(this[i] < pivot)
            i++
        while(this[j] > pivot)
            j--

        if(i <= j) {
            this.swap(i, j)
            i++
            j--
        }
    }

    if (low < j)
        this.quicksort(low, j)
    if (i < high)
        this.quicksort(i, high)
}