Power of Four，poweroffour

Power of Four，poweroffour

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?

4的冪次方 二進制裡只有1個1

判斷二進制裡有沒有1個1 用n & (n-1)== 0    

1 bool isPowerOfFour(int num) {
2     if(num <= 0) return false;
3     if(num & (num - 1)) return false; 
4     if(num & 0x55555555) return true; // 再將不是 4 的 N 次方的數字去掉
5     return false;
6 }