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; }