[Swift]LeetCode89. 格雷编码 | Gray Code

★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公众号：山青咏芝（shanqingyongzhi）
➤博客园地址：山青咏芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：https://www.cnblogs.com/strengthen/p/9936181.html 
➤如果链接不是山青咏芝的博客园地址，则可能是爬取作者的文章。
➤原文已修改更新！强烈建议点击原文地址阅读！支持作者！支持原创！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★

热烈欢迎，请直接点击！！！

进入博主App Store主页，下载使用各个作品！！！

注：博主将坚持每月上线一个新app！！！

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.

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.
             Therefore, for n = 0 the gray code sequence is [0].

---

格雷编码是一个二进制数字系统，在该系统中，两个连续的数值仅有一个位数的差异。

给定一个代表编码总位数的非负整数 n，打印其格雷编码序列。格雷编码序列必须以 0 开头。

示例 1:

输入: 2
输出: [0,1,3,2]
解释:
00 - 0
01 - 1
11 - 3
10 - 2

对于给定的 n，其格雷编码序列并不唯一。
例如，[0,2,3,1] 也是一个有效的格雷编码序列。

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

示例 2:

输入: 0
输出: [0]
解释: 我们定义格雷编码序列必须以 0 开头。
     给定编码总位数为 n 的格雷编码序列，其长度为 2n。当 n = 0 时，长度为 20 = 1。
     因此，当 n = 0 时，其格雷编码序列为 [0]。

---

8ms

class Solution {
    func grayCode(_ n: Int) -> [Int] {
        var result = [Int]()
        var i: Int = 0
        while i < (1 << n) {
            result.append( i ^ (i >> 1))
            i += 1
        }
        return result
    }
}

---

8ms

class Solution {
    func grayCode(_ n: Int) -> [Int] {
        var result = [Int]()
        for i in 0..<1 << n {
            result.append((i >> 1) ^ i)
        }
        return result
    }
}

---

12ms

class Solution {
    func grayCode(_ number: Int) -> [Int] {
        guard number > 0 else {
            return [0]
        }
        var result: [Int] = [0, 1]
        for idx in 1..<number {
            for iidx in (0...result.count - 1).reversed() {
                result.append(result[iidx] + 1 << idx)
            }
        }
        return result
    }
}

---

16ms

class Solution {
    func grayCode(_ n: Int) -> [Int] {
        if n == 0 {
            return [0]
        }
        
        if n == 1 {
            return [0, 1]
        }
        
        var res = [0, 1]
        
        for i in 1..<n {
            var result: [Int] = []
            for val in res.reversed() {
                result.append(val + (1<<i))
            }
            res = res + result
        }
        
        return res
    }
}

---

24ms

class Solution {
    func grayCode(_ n: Int) -> [Int] {
        if n == 0 {
            return [0]
        }
        var result = [Array<Int>(repeating: 0, count: n)];
        for i in 0 ..< n {
            let count = result.count
            for j in stride(from: count - 1, to: -1, by: -1) {
                var temp = result[j]
                temp[n - i - 1] = 1
                result.append(temp)
            }
        }
        var realResult = Array<Int>()
        for tempArr in result {
            realResult.append(binTodec(number: tempArr))
        }
        return realResult
    }
    
    func binTodec(number num: Array<Int>) -> Int {
        var sum: Int = 0
        for c in num {
            sum = sum * 2 + c
        }
        return sum
    }
}