GrayCode.java

import java.util.ArrayList;
import java.util.List;

/**
 * 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.
 * 
 * Reference: http://fisherlei.blogspot.com/2012/12/leetcode-gray-code.html
 * 
 * @author joshluo
 * 
 */
public class GrayCode {

    public List<Integer> grayCode(int n) {
        assert (n >= 0 && n <= 31);
        List<Integer> result = new ArrayList<Integer>();
        result.add(0);
        for (int i = 0; i < n; i++) {
            int highestBit = 1 << i;
            for (int j = result.size() - 1; j >= 0; j--) {
                result.add(highestBit + result.get(j));
            }
        }
        return result;
    }

}