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.

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

 1 class Solution {
 2 public:
 3     vector<int> grayCode(int n) {
 4         vector<int> res((1 << n), 0);
 5         for(int i = 0; i < (1 << n); ++i) {
 6             res[i] = (i >> 1) ^ i;
 7         }
 8         return res;
 9     }
10 };