# Gray Code

## Question

### Problem Statement

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, find the sequence of gray code. A gray code sequence must begin with 0 and with cover all $2^n$ integers.

#### 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.

[0,2,3,1] is also a valid gray code sequence according to the above definition.

#### Challenge

$O(2^n)$ time.

## 题解

### Java

public class Solution {
/**
* @param n a number
* @return Gray code
*/
public ArrayList<Integer> grayCode(int n) {
if (n < 0) return null;

ArrayList<Integer> currGray = new ArrayList<Integer>();

for (int i = 0; i < n; i++) {
int msb = 1 << i;
// backward - symmetry
for (int j = currGray.size() - 1; j >= 0; j--) {
}
}

return currGray;
}
}


## Reference

• Soulmachine 的 leetcode 题解