342. Power of Four

题目来源【Leetcode】

Given an integer (signed 32 bits), write a function to check whether it is a power of 4.

Example:
Given num = 16, return true. Given num = 5, return false.

Follow up: Could you solve it without loops/recursion?

方法一：取对数

class Solution {public:    bool isPowerOfFour(int num) {        if(num < 1) return false;        double i = log10(num)/log10(4);        return (i - (int)i) == 0;    }};

方法二：二进制特点：

class Solution {public:    bool isPowerOfFour(int num) {        return ((num-1)&num)==0 && (num-1)%3==0;    }};