The gray code is a binary numeral system where two successive values differ in only one bit.

Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code.
A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:

00 - 0
01 - 1
11 - 3
10 - 2
Note:
For a given n, a gray code sequence is not uniquely defined.

For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.

For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.

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

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