0231. Power of Two

# 231. Power of Two#

## 题目 #

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

Example 1:

``````Input: 1
Output: true
Explanation: 2^0 = 1
``````

Example 2:

``````Input: 16
Output: true
Explanation: 2^4 = 16
``````

Example 3:

``````Input: 218
Output: false
``````

## 解题思路 #

• 判断一个数是不是 2 的 n 次方。
• 这一题最简单的思路是循环，可以通过。但是题目要求不循环就要判断，这就需要用到数论的知识了。这一题和第 326 题是一样的思路。

## 代码 #

``````
package leetcode

// 解法一 二进制位操作法
func isPowerOfTwo(num int) bool {
return (num > 0 && ((num & (num - 1)) == 0))
}

// 解法二 数论
func isPowerOfTwo1(num int) bool {
return num > 0 && (1073741824%num == 0)
}

// 解法三 打表法
func isPowerOfTwo2(num int) bool {
allPowerOfTwoMap := map[int]int{1: 1, 2: 2, 4: 4, 8: 8, 16: 16, 32: 32, 64: 64, 128: 128, 256: 256, 512: 512, 1024: 1024, 2048: 2048, 4096: 4096, 8192: 8192, 16384: 16384, 32768: 32768, 65536: 65536, 131072: 131072, 262144: 262144, 524288: 524288, 1048576: 1048576, 2097152: 2097152, 4194304: 4194304, 8388608: 8388608, 16777216: 16777216, 33554432: 33554432, 67108864: 67108864, 134217728: 134217728, 268435456: 268435456, 536870912: 536870912, 1073741824: 1073741824}
_, ok := allPowerOfTwoMap[num]
return ok
}

// 解法四 循环
func isPowerOfTwo3(num int) bool {
for num >= 2 {
if num%2 == 0 {
num = num / 2
} else {
return false
}
}
return num == 1
}

``````

Apr 8, 2023