342. Power of Four

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?

判断一个数是不是4的次方

C++(6ms):

class Solution {
public:
    bool isPowerOfFour(int num) {
        return (num&(num-1))==0 && (num-1)%3==0 ;
    }
};

C++（9ms）：

class Solution {
public:
    bool isPowerOfFour(int num) {
        if (num&(num-1))
            return false ;
        if (num < 0)
            return false ;    
        while(num){
            if (num&1)
                return true ;
            num>>=2 ;
        }
        return false ;
    }
};

java(3ms):

class Solution {
    public boolean isPowerOfFour(int num) {
        return (num&(num-1))==0 && (num-1)%3==0 ;
    }
}