Leetcode之Gray Code

题目：

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.

Example 1:

Input: 2
Output: [0,1,3,2]
Explanation:
00 - 0
01 - 1
11 - 3
10 - 2

For a given n, a gray code sequence may not be uniquely defined.
For example, [0,2,3,1] is also a valid gray code sequence.

00 - 0
10 - 2
11 - 3
01 - 1

Example 2:

Input: 0
Output: [0]
Explanation: We define the gray code sequence to begin with 0.
             A gray code sequence of n has size = 2n, which for n = 0 the size is 20 = 1.
             Therefore, for n = 0 the gray code sequence is [0].

代码：

class Solution {
public:
    vector<int> grayCode(int n) {
        vector<int> res;
	if(n < 0)return res;
	if (n == 0) {
		res.push_back(0);
		return res;
	}

	vector<int> last = grayCode(n - 1);
    
	for (int i = 0; i < last.size(); i++) {
		res.push_back(last[i]);
	}
	for (int i = last.size() - 1; i >= 0; i--) {
		int num = last[i] + pow(2, n - 1);
		res.push_back(num);
	}
	return res;
    }
};

想法：

运用递归