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.

思路：一开始数组中只有0，然后在二进制形势下，反复把结果数组中所有的已知结果最高位的1前面的0置为1，这样做结果肯定只相差改变的这一位。

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