Check Power of Three

Problem

Given a positive integer num, return true if num is a power of three, and false otherwise.

Examples

isPowerOfThree(81) 
// returns true
// 81 is a power of three (3^4)

isPowerOfThree(9) 
// returns true
// 9 is a power of three (3^2)

isPowerOfThree(2) 
// returns false
// 2 is not a power of three