func mergeSort(array: [Int]) -> [Int ] {

    print(array)

    guard array.count > 1 else {
        return array
    }

    let midpoint = array.count/2

    let leftPile = self.mergeSort(array: Array(array[0..<midpoint]))
    let rightPile = self.mergeSort (array: Array(array[midpoint..<array.count]))
    return merge(leftPile: leftPile, rightPile: rightPile)
}