leetcode--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, 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 class Solution {
    /**We start from 00...0 and 000...1. Changing i-th bit for each for them is same as to 
	 * add pow(2, i) to them. 	 * 
	 * @param n  --Integer, the length of the binary string
	 * @return List<Integer> --all possible 
	 * @author Averill Zheng
	 * @version 2014-06-05
	 * @since JDK 1.7
	 */
	public List<Integer> grayCode(int n) {
		List<Integer> result = new ArrayList<Integer>();
		result.add(0);
		if(n > 0){
			result.add(1);
			int length = 2;
			for(int i = 1; i < n; ++i){
				for(int j = length - 1; j > -1; --j)
					result.add(result.get(j) + (int)Math.pow(2, i));
				length *= 2;
			}
		}
		return result;
    }
}