【Leetcode】Power of Four

题目链接：https://leetcode.com/problems/power-of-four/

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.

Follow up: Could you solve it without loops/recursion?

思路：

观察4个幂次方，发现除了最高位，都是0，且0的个数是偶数，如4=100 16=10000, 8=1000。否则就不是4的power

算法：

public static boolean isPowerOfFour(int num) {
        if(num==1)
            return true;
        if(num==2||num==3)
            return false;
        if (num<1)
            return false;  

        String binary = Integer.toBinaryString(num);
        char c[] = binary.toCharArray();
        int count = 0;  

        for(int i=1;i<c.length;i++){//除了开头全是0
            if(c[i]!='0'){
                return false;
            }else{//统计0的个数
                count++;
            }
        }  

        if(count!=0&&count%2==0)//有偶数个0
            return true;
        else
            return false;
}