2.1.18Gray Code

Notes:
	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.
	Solution: Refer to http://en.wikipedia.org/wiki/Gray_code.

class Solution {
public:
    vector<int> grayCode(int n) {
        vector<int> result(1 << n, 0);
        for (int i = 0; i < 1 << n; ++i)
            result[i] = (i >> 1) ^ i;//按位异或 
        return result;
    }
};