Leetcode | 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.

注意一下初始化。

然后每增加一位，其实就是就是加上（最高位置为1然后反向的那一部分）。最高位为0的已经有了。

比如1位的数： 

0

1

2位的数就是先保留原来的数：

00

01

然后反向地为每个数最高位（第二位）置1：

11

10

所以2位的gray code就是：

00 01 11 10

3位就是：

00 01 11 10->（原来的）000 001 011 010

->（反向高位置1的）110 111 101 100->（总的）000 001 011 010 110 111 101 100

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