89. Gray Code
Description
An nbit gray code sequence is a sequence of 2^{n}
integers where:
 Every integer is in the inclusive range
[0, 2^{n}  1]
,  The first integer is
0
,  An integer appears no more than once in the sequence,
 The binary representation of every pair of adjacent integers differs by exactly one bit, and
 The binary representation of the first and last integers differs by exactly one bit.
Given an integer n
, return any valid nbit gray code sequence.
Example 1:
Input: n = 2 Output: [0,1,3,2] Explanation: The binary representation of [0,1,3,2] is [00,01,11,10].  00 and 01 differ by one bit  01 and 11 differ by one bit  11 and 10 differ by one bit  10 and 00 differ by one bit [0,2,3,1] is also a valid gray code sequence, whose binary representation is [00,10,11,01].  00 and 10 differ by one bit  10 and 11 differ by one bit  11 and 01 differ by one bit  01 and 00 differ by one bit
Example 2:
Input: n = 1 Output: [0,1]
Constraints:
1 <= n <= 16
Solutions
Solution 1: Binary to Gray Code Conversion
Gray code is a type of encoding method that we often encounter in engineering. Its basic feature is that only one bit of binary number is different between any two adjacent codes.
The rule for converting binary code to binary Gray code is to keep the highest bit of the binary code as the highest bit of the Gray code, and the second highest bit of the Gray code is the XOR of the highest bit and the second highest bit of the binary code. The calculation of the remaining bits of the Gray code is similar to the second highest bit.
Assume that a binary number is represented as $B_{n1}B_{n2}…B_2B_1B_0$, and its Gray code is represented as $G_{n1}G_{n2}…G_2G_1G_0$. The highest bit is retained, so $G_{n1} = B_{n1}$; and the other bits $G_i = B_{i+1} \oplus B_{i}$, where $i=0,1,2..,n2$.
Therefore, for an integer $x$, we can use the function $gray(x)$ to get its Gray code:


We directly map the integers $[0,..2^n  1]$ to the corresponding Gray codes to get the answer array.
The time complexity is $O(2^n)$, where $n$ is the integer given in the problem. Ignoring the space consumption of the answer, the space complexity is $O(1)$.









