0: 0

1: 0 || 1

2: 0 1 || 11 10

3: 0 1 11 10 || 110 111 101 100

Add 1 bit in front of each number in the existing list in reverse order.

```
public class Solution {
public List<Integer> grayCode(int n) {
List<Integer> res = new ArrayList<>();
res.add(0);
if (n <= 0) return res;
for (int i = 1; i <= n; i++) {
int size = res.size();
for (int j = size - 1; j >= 0; j--) {
res.add(res.get(j) + (1 << (i - 1)));
}
}
return res;
}
}
```