LeetCode - Gray Code

Gray Code

//
public List<Integer> grayCode(int n) {
        ArrayList<Integer> ret = new ArrayList<Integer>();
        if(n < 0) return ret;
        ret.add(0);
        if(n == 0) return ret;
        for(int i=0; i<n; i++){
            ArrayList<Integer> temp = new ArrayList<>();
            for(int j=ret.size()-1; j>=0; j--){
                temp.add((1<<i) + ret.get(j)); // the priority of +/- operation is higher then >>/<< shift operation
            }
            ret.addAll(temp);
        }
        return ret;
    }