leetcode 326 [easy]--- Power of Three

难度：easy

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

思路：判断一个整数是否为3的幂。用while循环让整数一直被3除，如果最后整数为1，则它就是3的幂。