func rotLeft(a: [Int], d: Int) -> [Int] {
    var array: [Int] = []
    for i in 0 ..< a.count {
        let index = (i + d) % a.count
        array.append(a[index])
    }
    return array
}


rotLeft(a: [1,2,3,4,5], d: 4) // output [5, 1, 2, 3, 4]