326. Power of Three

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?

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

这里可以用log的换底公式，logab = logb/loga。代码如下：

public class Solution {    public boolean isPowerOfThree(int n) {        double num = Math.log10(n) / Math.log10(3);        if (num % 1 == 0) {            return true;        } else {            return false;        }    }}

这里如果用Math.log的话，测试集243结果是4.99999999，这跟计算机组成原理有关。

也可以用下面这个思路，

public class Solution {    public boolean isPowerOfThree(int n) {        // 1162261467 is 3^19,  3^20 is bigger than int          return ( n>0 &&  1162261467%n==0);    }}