342. Power of Four

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?

思路：

首先必须是2的平方，其次减去1之后可以被3整除。

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