[Swift]LeetCode231. 2的冪 | Power of Two

★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公衆號：山青詠芝（shanqingyongzhi）
➤博客園地址：山青詠芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：http://www.javashuo.com/article/p-vyhqcjqi-mc.html 
➤若是連接不是山青詠芝的博客園地址，則多是爬取做者的文章。
➤原文已修改更新！強烈建議點擊原文地址閱讀！支持做者！支持原創！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★

Given an integer, write a function to determine if it is a power of two.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

---

給定一個整數，編寫一個函數來判斷它是不是 2 的冪次方。

示例 1:

輸入: 1
輸出: true
解釋: 20 = 1

示例 2:

輸入: 16
輸出: true
解釋: 24 = 16

示例 3:

輸入: 218
輸出: false

---

12ms

 1 class Solution {
 2     func isPowerOfTwo(_ n: Int) -> Bool {
 3         for i in 0..<32 {
 4             if n == 1 << i {
 5                 return true
 6             }
 7         }
 8         
 9         return false
10     }
11 }

---

12ms

 1 class Solution {
 2     func isPowerOfTwo(_ n: Int) -> Bool {
 3         if n <= 0 { return false }
 4         if n == 1 { return true }
 5         var current = n
 6         while current > 2 {
 7             if current % 2 != 0 {
 8                 return false
 9             } else {
10                 current = current / 2
11             }
12         }
13         return true
14     }
15 }

---

16ms

1 class Solution {
2     func isPowerOfTwo(_ n: Int) -> Bool {
3         guard n > 0 else { return false }
4         return String(n, radix: 2).filter { $0 == "1" }.count == 1
5     }
6 }

---

20ms

 1 class Solution {
 2     func isPowerOfTwo(_ n: Int) -> Bool {
 3         if (n <= 0) {
 4             return false
 5         }
 6         var rencontre = false
 7         for digit in String(n, radix: 2) {
 8             if (digit == "1") {
 9                 if rencontre {
10                     return false
11                 } else {
12                     rencontre = true
13                 }
14             }
15         }
16         
17         return true
18     }
19 }

---

20ms

 1 class Solution {
 2     func isPowerOfTwo(_ n: Int) -> Bool {
 3         
 4         guard n > 0 else {
 5             return false
 6         }
 7         
 8         var num = n
 9         
10         while num != 0 {
11             
12             if num == 1 || num == 2 {
13                 return true
14             }
15             
16             if num % 2 != 0 {
17                 return false
18             }
19             
20             num /= 2
21         }
22         
23         return true
24         
25     }
26 }

---

24ms

1 class Solution {
2     func isPowerOfTwo(_ n: Int) -> Bool {
3         if (n == 0) {
4             return false;
5         }
6         return n & (n-1) == 0
7     }
8 }

---

32ms

 1 class Solution {
 2     func isPowerOfTwo(_ n: Int) -> Bool {
 3         var number:Int = n;
 4         if number != 0 && number > 0 {
 5             if number == 1 || number == 2{
 6                 return true;
 7             }
 8             while (number%2 == 0) {
 9                 if (number == 2) {
10                     break;
11                 }
12                 number = number / 2;
13             }
14             if (number % 2 == 0) {
15                 return true;
16             }
17         }
18         return false;
19     }
20 }

---

48ms

1 class Solution {
2     func isPowerOfTwo(_ n: Int) -> Bool {
3         if n <= 0 {
4             return false
5         }
6         return 1<<32 % n == 0
7     }
8 }