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.

class Solution {public:    vector<int> grayCode(int n) {        vector<int> result;        result.push_back(0);        for (int i = 0; i < n; ++i) {            const int highest_bit = 1 << i;            for (int j = result.size() - 1; j >= 0; --j) {                result.push_back(highest_bit | result[j]);            }        }        return result;    }};