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?

一刷
题解：保证二进制的形式只有一个1，并且1出现在奇数位。

class Solution {
    public boolean isPowerOfFour(int num) {
        return num>0 &&(num&(num-1)) == 0 &&(num & 0x55555555) !=0;
         //so that the single 1 bit always appears at the odd position 
        //0x55555555 is to get rid of those power of 2 but not power of 4
    }
}