  [LeetCode][Medium] 89. Gray Code
2021-04-06
Engineering
2 MIN

## 題目

An n-bit gray code sequence is a sequence of `2n` integers where:

• Every integer is in the inclusive range `[0, 2n - 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 n-bit 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]
``````

## 思路

• n = 0 
• n = 1 [0, 1]
• n = 2 [0, 1, 3, 2]
• n = 3 [0, 1, 3, 2, 6, 7, 5, 4]

## Solution

``````/**
* @param {number} n
* @return {number[]}
*/
var grayCode = function(n) {
var res = 
var count = 0
while(count < n) {
const offset = Math.pow(2, n - 1)
const temp = [...res].reverse().map(v => v + offset)
res = res.concat(temp)
n--
}
return res
};
``````

## Complexity

• Time complexity : O(n)
• Space complexity : O(1)