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?

Credits:

Special thanks to for adding this problem and creating all test cases.

给一个有符号的32位整数，写一个函数检查此数是否为4的次方数。

解法1：位操作

解法2：循环

解法3: 数学函数, 换底公式

Java:

public boolean isPowerOfFour(int num) {    int count0=0;    int count1=0;     while(num>0){        if((num&1)==1){            count1++;        }else{            count0++;        }         num>>=1;    }     return count1==1 && (count0%2==0);}　　

Java:

public boolean isPowerOfFour(int num) {    while(num>0){        if(num==1){            return true;        }         if(num%4!=0){            return false;        }else{            num=num/4;        }    }     return false;}

Java:

public boolean isPowerOfFour(int num) {   if(num==0) return false;    int pow = (int) (Math.log(num) / Math.log(4));   if(num==Math.pow(4, pow)){       return true;   }else{       return false;   }}

Python:

class Solution(object):    def isPowerOfFour(self, num):        """        :type num: int        :rtype: bool        """        return num > 0 and (num & (num - 1)) == 0 and \               ((num & 0b01010101010101010101010101010101) == num)

Python:

# Time:  O(1)# Space: O(1)class Solution2(object):    def isPowerOfFour(self, num):        """        :type num: int        :rtype: bool        """        while num and not (num & 0b11):            num >>= 2        return (num == 1)

C++:

class Solution {public:    bool isPowerOfFour(int num) {        while (num && (num % 4 == 0)) {            num /= 4;        }        return num == 1;    }};

C++:

class Solution {public:    bool isPowerOfFour(int num) {        return num > 0 && int(log10(num) / log10(4)) - log10(num) / log10(4) == 0;    }};

C++:

class Solution {public:    bool isPowerOfFour(int num) {        return num > 0 && !(num & (num - 1)) && (num & 0x55555555) == num;    }};

C++:　　

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

　　　

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