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.

要求 n == 0 时，返回 0， 有点奇怪。

class Solution {
public:
vector<int> grayCode(int n) {
vector<int> ivec = { 0 };
if (n <= 0)
return ivec;
ivec.push_back(1);
int num = 1;
for (int i = 1; i < n; i++) {
num *= 2;
for (int j = ivec.size() - 1; j >= 0; j--) {
ivec.push_back(ivec[j] + num);
}
}
return ivec;
}
};