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public class Solution {
    public boolean isPowerOfThree(int n) {
        return (n > 0) && ((int)Math.pow(3, 19) % n == 0);
        
    }
}

//因为3是质数、同理5，7也可以用这个方法，但是9不行
// 1162261467 is 3^19,  3^20 is bigger than int  

class Solution {
public:
    bool isPowerOfThree(int n) {
        return fmod(log10(n)/log10(3), 1)==0;
    }
};