326. Power of Three

题目来源【Leetcode】

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

Follow up:
Could you do it without using any loop / recursion?

这道题要求不用循环来判断这个数是否为3的N次方

方法一： 用取对数的办法 n = 3^i;最后就是证明log10(n)/log10(3)是否为整数

class Solution {
public:
    bool isPowerOfThree(int n) {
        double i = log10(n)/log10(3);
        return ((i-(int)i)== 0);
    }
};

方法二：根据能够表示最大3的N次方的数来除n：

class Solution {
public:
    bool isPowerOfThree(int n) {
        return n > 0 && 1162261467%n == 0 ;
    }
};