Gray Code

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.

Have you met this question in a real interview? 

Yes

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.

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

Challenge

O(2n) time.

public class Solution {

    /**

     * @param n a number

     * @return Gray code

     */

    public ArrayList<Integer> grayCode(int n) {

        // Write your code here

        int size = 1 << n;

        ArrayList<Integer> res = new ArrayList<Integer>();

        for(int i = 0; i < size; i++){

            res.add((i >> 1) ^ i);

        }

        return res;

    }

}