# LeetCode Gray Code

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:
void work(vector<int>& ans, unordered_set<int>& codes, int gray, int n) {
for (int i = 0; i < n; ++i) {
int new_gray = (gray&(~(1 << i))) | (~gray)&(1 << i); // flip i-th bit
if (codes.find(new_gray) == codes.end()) {
codes.insert(new_gray);
ans.push_back(new_gray);
work(ans, codes, new_gray, n);
break;
}
}
}
vector<int> grayCode(int n) {
vector<int> ans = { 0 };
if (n == 0)return ans;
unordered_set<int> codes = { 0 };
work(ans, codes, 0, n);
return ans;
}
};
```

```class Solution {
public:
vector<int> grayCode(int n) {
vector<int> ans = { 0 };
if (n == 0)return ans;
ans.push_back(1);
for (int i = 2; i <= n; ++i) {
int sz = ans.size();
while (sz--) {
ans.push_back(ans[sz] | (1 << (i - 1)));
}
}
return ans;
}
};
```