Gray Code

 1 public class Solution {
    public ArrayList<Integer> grayCode(int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(n < 0)
            return result;
        int count = 1 << n;
        for(int i = 0; i < count; i++)
            result.add(i^(i>>1));
        return result;
        
    }
}

public class Solution {
    public ArrayList<Integer> grayCode(int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(n <= 1){
            result.add(0);
            if(n == 1){
                result.add(1);
            }
            return result;
        }
        
        ArrayList<Integer> pre = grayCode(n - 1);
        int highest = 1 << (n - 1);
        result.addAll(pre);
        for(int i = pre.size() - 1; i >= 0 ; i --){
            result.add(pre.get(i) + highest);
        }
        return result;
    }
}