Power-of-Three.md

##Power of Three

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?

##思路1

• 各种方法集锦

public class Solution {
    public boolean isPowerOfThree(int n) {
        if (n <= 0) {
            return false;
        }

        while (n > 1) {
            if (n / 3 * 3 != n) {
                return false;
            }
            n /= 3;
        }

        return true;
    }
}

##思路2

public class Solution {
    public boolean isPowerOfThree(int n) {
        if (n <= 0) {
            return false;
        }

        long i = 1;
        while (i <= n) {
            if (i == n) {
                return true;
            }
            i *= 3;
        }

        return false;
    }
}

####思路3

public class Solution {
    public boolean isPowerOfThree(int n) {
        // 1162261467 is 3^19,  3^20 is bigger than int
        return ( n>0 &&  1162261467%n==0);
    }
}

##最简单方法

###对数换底公式

				logn(b)
	loga(b) = 	-------
				logn(a)

public class Solution {
    public boolean isPowerOfThree(int n) {
        return (Math.log10(n) / Math.log10(3)) % 1 == 0;
    }
}