（难度Medium）Problem 89. Gray Code（灰色代码）

### 链接

Gray Code - LeetCode

Medium

### 题干

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.

### 样例

Example 1:

**Input:** 2
**Output:** [0,1,3,2]
**Explanation:**
00 - 0
01 - 1
11 - 3
10 - 2

For a given *n*, a gray code sequence may not be uniquely defined.
For example, [0,2,3,1] is also a valid gray code sequence.

00 - 0
10 - 2
11 - 3
01 - 1

Example 2:

`
Input: 0
Output: [0]
Explanation: We define the gray code sequence to begin with 0.
A gray code sequence of n has size = 2n, which for n = 0 the size is 20 = 1.