leetcode_326. Power of Three-判断是否3的次方

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?

判断一个数是否是3的次方？

法1：网友神奇方法

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

法2：借助对数函数

    // 借助对数函数，注意，对数函数在求log(243,3)时得到4.999999999999999而非5，所以要注意    public boolean isPowerOfThree(int n) {        if(n<=0) return false;        double d = log(n, 3);        int t = (int) Math.floor(d+0.1);        return n==Math.pow(3, t);    }    public double log(double value, double base) {        return Math.log(value) / Math.log(base);    }