【leetcode】342. Power of Four

題目：
Given an integer (signed 32 bits), write a function to check whether it is a power of 4.

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思路：
(num&(num-1))==0保證整個二進制中只有一個1；(num-1) % 3 == 0保證1位於二進制中的偶數位上。

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代碼實現：

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

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參考：
https://leetcode.com/problems/power-of-four/discuss/80460/1-line-C%2B%2B-solution-without-confusing-bit-manipulations