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.

分析：

和判斷一個數是否為3的冪類似，可以用迴圈/4判斷餘數或者利用對數來判斷，程式碼如下：

解法1：

class Solution {
public:
    bool isPowerOfFour(int num) {
        if(num <= 0)
            return false;
        if(num == 1)
            return true;
        while(num/4 != 1)
        {
            if(num%4 != 0)
                return false;
            else
               num /= 4; 
        }
        if(num % 4 == 0)
            return true;
        else
            return false;    
    }
};
 

解法2：

class Solution {
public:
    bool isPowerOfThree(int num) {
        if(num <= 0)
            return false;
       if(int(log10(num)/log10(4)) - log10(num)/log10(4) == 0)
          return true;
       else
          return false;   
    }
};