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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?

## 解题思路:

1. 用数学公式$\frac{log_{10}n}{log_{10}3}$为整数
2. 更多参考:http://blog.csdn.net/ebowtang/article/details/50485622