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 - 001 - 111 - 310 - 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:

class Solution{public:    /*     * for reference: http://en.wikipedia.org/wiki/Gray_code.     */    vector<int> grayCode(int n)    {        vector<int> ret;        int size = 1 << n;        for(int i = 0; i < size; ++i)            ret.push_back((i >> 1)^i);        return ret;    }};