// 1st way
func grayCode(_ n: Int) -> [Int] {
    var res = [Int]()
    res.append(0)

    for i in 1...n {
        let size = res.count
        let mask = 1<<(i-1)
        for j in stride(from: size-1, through: 0, by: -1) {
            res.append(mask | res[j])
        }
    }

    return res
}

// 2nd way
func grayCode(_ n: Int) -> [Int] {
    var res = [Int]()
    let len = 1<<nß
    for i in 0..<len {
        let num = i ^ (i >> 1)
        res.append(num)
    }
    return res
}