0089. Gray Code

class Solution {

    public List<Integer> grayCode(int n) {
        List<Integer> arr = new ArrayList<>();
        arr.add(0);
        arr.add(1);
        for (int power = 2; power <= n; power++) {
            for (int i = arr.size() - 1; i > -1; i--) {
                arr.add(arr.get(i) + (int)Math.pow(2, power - 1));
            }
        }
        return arr;
    }
}