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.

原题链接：https://oj.leetcode.com/problems/gray-code/

题目：对给定的n求格雷码。

第n位的格雷码由两部分构成，一部分是n-1位格雷码，再加上 1<<(n-1)和n-1位格雷码的逆序的和。

	public List<Integer> grayCode(int n) {
		if (n == 0) {
			List<Integer> result = new ArrayList<Integer>();
			result.add(0);
			return result;
		}

		List<Integer> tmp = grayCode(n - 1);
		int addNumber = 1 << (n - 1);
		List<Integer> result = new ArrayList<Integer>(tmp);
		for (int i = tmp.size() - 1; i >= 0; i--) {
			result.add(addNumber + tmp.get(i));
		}
		return result;
	}

reference : http://blog.csdn.net/fightforyourdream/article/details/14517973