518. Coin Change 2

    //f(x) = n
    //拼出x块钱可以的方法数
    func change(_ amount: Int, _ coins: [Int]) -> Int {
        var arr = [Int](repeating: 0, count: amount + 1)
        arr[0] = 1
        for coin in coins{
            for i in stride(from: 1, to: amount + 1, by:1){
                if i - coin >= 0{
                    arr[i] += arr[i - coin]
                }
            }
        }
        return arr[amount]
    }