Kotlin Playground - Display which numbers are in array 1, but not array 2

ReconcileHelper.kt

// My solution to: https://pl.kotl.in/Q1PY9OvAu
//
// Write a function called "reconcileHelper" that processes two arrays of integers.
// Each array will have only distinct numbers (no repeats of the same integer in 
// the same array), and the arrays are not sorted.
// Your job is to find out which numbers are in array 1, but not array 2, 
// and which numbers are in array 2, but not in array 1.

// Your function should return a string formatted as so:
// "Numbers in array 1 that aren't in array 2: 
// <num1> <num2> <num3>...
// 
// Numbers in array 2 that aren't in array 1: 
// <num1> <num2> <num3>...
// "
//
// Each list of numbers should be listed in ascending order (lowest to largest)
// 
// So for instance, if I passed in: 
// reconcileHelper([5, 3, 4], [4, 3, 10, 6])
//
// The function would return this multiline string:
// "Numbers in array 1 that aren't in array 2: 
// 5
//
// Numbers in array 2 that aren't in array 1: 
// 6 10
// "
//
// Notes: 
// 1) You are allowed to use any standard library functions, but if it has a way of 
//        doing this EXACT problem, please don't use it. 
//        For example, don't use a function to subtract one array from another. That's too easy.
// 2) Try to make your solution fast so that it can handle over 1,000,000 elements in each array. 
//    (Doing a linear search through array `b`, for every element in array `a` will work, but is too slow.)
// 3) We don’t anticipate this taking you more than 30 minutes or so, but there is no hard limit.

fun Array<Int>.filterANotB(secondArray: Array<Int>): String {
    val listOfANotB = mutableListOf<Int>()
    forEach {
        if (!secondArray.contains(it)) {
            listOfANotB.add(it)
        }
    }
    return listOfANotB.sorted().toString().replace("[]\\[,]".toRegex(), "")
}

fun reconcileHelper(a : Array<Int>, b: Array<Int>) : String {
    val array1 = a.filterANotB(b)
    val array2 = b.filterANotB(a)
    
    return "Numbers in array 1 that aren't in array 2:\n$array1\n\nNumbers in array 2 that aren't in array 1:\n$array2"
}

fun main() {
    val result = reconcileHelper(arrayOf(5, 3, 4), arrayOf(4, 3, 10, 6))
    
    // Should print:
    // Numbers in array 1 that aren't in array 2: 
    // 5
    // 
    // Numbers in array 2 that aren't in array 1: 
    // 6 10
    println(result)
}