[LeetCode]--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.

前提学习：
Java Math的floor，round，ceil函数小结

想法：

3^x=n

log(3^x) = log(n)

x log(3) = log(n)

x = log(n) / log(3)

We need to

largest number power of 3, is 1162261467, we can use this to calculate the float precision. 0.0000000001 at least

Math.log(1162261468)/log(3)

private static final double epsilon = 10e-15;

    public boolean isPowerOfThree(int n) {
        if (n == 0)
            return false;
        double res = Math.log(n) / Math.log(3);
        return Math.abs(res - Math.round(res)) < epsilon;
    }

常规办法（AC过的）：

public boolean isPowerOfThree(int n) {
        if (n == 0)
            return false;
        if (n == 1)
            return true;
        if (n % 3 == 0)
            return isPowerOfThree(n / 3);
        return false;
    }