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, find the sequence of gray code. A gray code sequence must begin with 0 and with cover all 2n integers.

For a given n, a gray code sequence is not uniquely defined.

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

Example

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

00 - 0
01 - 1
11 - 3
10 - 2
public class Solution {
    public ArrayList<Integer> grayCode(int n) {
        int count = (int)Math.pow(2, n);
        ArrayList<Integer> result = new ArrayList<Integer>(count);
        for(int i = 0; i < count; i++)
            result.add(i ^ (i / 2)); //nth code = n xor floor(n/2)
        return result;
    }
}