Given 2 integers `n`

and `start`

. Your task is return **any** permutation `p`

of `(0,1,2.....,2^n -1) `

such that :

`p[0] = start`

`p[i]`

and`p[i+1]`

differ by only one bit in their binary representation.`p[0]`

and`p[2^n -1]`

must also differ by only one bit in their binary representation.

**Example 1:**

Input:n = 2, start = 3Output:[3,2,0,1]Explanation:The binary representation of the permutation is (11,10,00,01). All the adjacent element differ by one bit. Another valid permutation is [3,1,0,2]

**Example 2:**

Input:n = 3, start = 2Output:[2,6,7,5,4,0,1,3]Explanation:The binary representation of the permutation is (010,110,111,101,100,000,001,011).

**Constraints:**

`1 <= n <= 16`

`0 <= start < 2 ^ n`

```
struct Solution;
impl Solution {
fn circular_permutation(n: i32, start: i32) -> Vec<i32> {
let mut res = vec![];
for i in 0..1 << n {
res.push(start ^ (i ^ i >> 1));
}
res
}
}
#[test]
fn test() {
let n = 2;
let start = 3;
let res = vec![3, 2, 0, 1];
assert_eq!(Solution::circular_permutation(n, start), res);
let n = 3;
let start = 2;
let res = vec![2, 3, 1, 0, 4, 5, 7, 6];
assert_eq!(Solution::circular_permutation(n, start), res);
}
```