---
title: 'LeetCode 326. Power of Three'
disqus: hackmd
---

# LeetCode 326. Power of Three

## Description

Given an integer n, return true if it is a power of three. Otherwise, return false.

An integer n is a power of three, if there exists an integer x such that n == 3^x^.

## Example

Input: n = 27
Output: true

Input: n = 0
Output: false

## Constraints

-2^31^ <= x <= 2^31^ - 1

## Answer

方法一，用while來除n直到n變成1為止，若其中有餘數就return false，若無餘數就return true。

//2021_11_30
bool isPowerOfThree(int n){
    if(n > 0 && n & 1){
        while(n != 1){
            if(n % 3 != 0){return false;}
            n /= 3;
        }
        return true;
    }
    return false;
}

X = log~3~n = log(n) / log(3)，且3^X^ = n，若是3的次方數，則以log的整數取出X再做次方計算必為原n。

//2022_03_12
bool isPowerOfThree(int n){
    float tmp = log(n)/log(3);
    return n>0 && (pow(3,tmp) == n);
}

## Link
https://leetcode.com/problems/power-of-three/
###### tags: `Leetcode`