[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.

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