alman williams algorithm in swift

main.swift
func mergeSort(_ array: [Int]) -> [Int] {
    guard array.count > 1 else { return array }
    let middleIndex = array.count / 2
    let leftArray = mergeSort(Array(array[0..<middleIndex]))
    let rightArray = mergeSort(Array(array[middleIndex..<array.count]))
    return merge(leftArray, rightArray)
}

func merge(_ left: [Int], _ right: [Int]) -> [Int] {
    var leftIndex = 0, rightIndex = 0
    var resultArray = [Int]()
    while leftIndex < left.count && rightIndex < right.count {
        if left[leftIndex] < right[rightIndex] {
            resultArray.append(left[leftIndex])
            leftIndex += 1
        } else {
            resultArray.append(right[rightIndex])
            rightIndex += 1
        }
    }
    return resultArray + Array(left[leftIndex..<left.count]) + Array(right[rightIndex..<right.count])
}
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