Swift Merge Sort

func mergeSort<T: Comparable>(_ array: [T]) -> [T] {
  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<T: Comparable>(_ left: [T], _ right: [T]) -> [T] {
  var leftIndex = 0
  var rightIndex = 0
 
  var orderedArray: [T] = []
 
  while leftIndex < left.count && rightIndex < right.count {
    let leftElement = left[leftIndex]
    let rightElement = right[rightIndex]
 
    if leftElement < rightElement {
      orderedArray.append(leftElement)
      leftIndex += 1
    } else if leftElement > rightElement {
      orderedArray.append(rightElement)
      rightIndex += 1
    } else { 
      orderedArray.append(leftElement)
      leftIndex += 1
      orderedArray.append(rightElement)
      rightIndex += 1
    }
  }
 
  while leftIndex < left.count {
    orderedArray.append(left[leftIndex])
    leftIndex += 1
  }
 
  while rightIndex < right.count {
    orderedArray.append(right[rightIndex])
    rightIndex += 1
  }
 
  return orderedArray
}