var mixedNums: [Int] = [3,1,2,4]

func sortArrayByParity(_ A: [Int]) -> [Int] {
    let returnArray: [Int] = []
    var copiedArray = A
    let lastEleOfCopiedArray = copiedArray.popLast()


    guard !copiedArray.isEmpty else {
        return []
    }

    switch lastEleOfCopiedArray! % 2 == 0 {
    case true:
       let evenElementArray = [lastEleOfCopiedArray] + returnArray
       return sortArrayByParity(evenElementArray as! [Int])
    case false:
        let oddElementArray = returnArray + [lastEleOfCopiedArray]
        return sortArrayByParity(oddElementArray as! [Int])
    }
}

sortArrayByParity(mixedNums)

//expected, I think?: [2,4,3,1]