Leetcode——326. Power of Three

题目

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?

解答

Method1：循环或者递归

public boolean isPowerOfThree(int n) {
return n>0 && (n==1 || (n%3==0 && isPowerOfThree(n/3)));
}

public boolean isPowerOfThree(int n) {
if(n>1)
while(n%3==0) n /= 3;
return n==1;
}

因为3是质数，所以可以用最大的3的幂次和n求模来计算。

最大的的3的幂次：

int MaxPowerOfThree=(int) pow(3,(int)(log10(INT_MAX)/log10(3)));

AC code

class Solution {
public:
bool isPowerOfThree(int n) {
int MaxPowerOfThree=(int) pow(3,(int)(log10(INT_MAX)/log10(3)));
return n>0&&MaxPowerOfThree%n==0;
}
};

Or simply hard code it since we know maxPowerOfThree = 1162261467:

public boolean isPowerOfThree(int n) {
return n > 0 && (1162261467 % n == 0);
}