[LeetCode] 89. 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.

// 3ms
class Solution {
public:
vector<int> grayCode(int n) {
vector<int> res;
int code = 0;

if (n > 0)
grayCode(res, code, n - 1);

return res;
}
private:
void grayCode(vector<int>& res, int& code, int nth_bit) {
if (nth_bit < 0) {
res.push_back(code);
return;
}

grayCode(res, code, nth_bit - 1);
code ^= (1 << nth_bit);
grayCode(res, code, nth_bit - 1);
}
};