Power of Four（Difficulty: Easy）

题目：

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.

实现：

class Solution {
public:
bool isPowerOfFour(int num) {
return (num > 0) && ((num & (num - 1)) == 0) && ((num & 0x55555555) == num);
}
};

分析：

(num & (num - 1)) == 0 判断这个数是不是2的倍数，(num & 0x55555555) == num 判断是不是4的倍数。0x55555555就是二进制数上奇数位上为1.