Gray Code

Problem

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.

Notice 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

Code - Java

public class Solution {
    /**
     * @param n a number
     * @return Gray code
     */
    public ArrayList<Integer> grayCode(int n) {
        ArrayList<Integer> result = new ArrayList<>();
        result.add(0);
        for (int i = 0; i < n; i++) {
            int len = result.size();
            int k = 1 << i;
            for (int j = len - 1; j >= 0; j--) {
                result.add(k + result.get(j));
            }
        }
        return result;
    }
}