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：
1 class Solution {
2 public:
3 bool isPowerOfFour(int num) {
4 return (num > 0) && ((num & (num - 1)) == 0) && ((num & 0x55555555) == num);
5 }
6 };
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.