326. Power of Three ,231. Power of Two ,

326. Power of Three

logn3=logn10log310

class Solution {public:bool isPowerOfThree(int n) {double logRes = log10(n) / log10(3);if (logRes - (int)logRes == 0){return true;}return false;}};

231. Power of Two

class Solution {public:    bool isPowerOfTwo(int n) {        if(n<=0){            return false;        }        return (n&(n-1))==0;    }};

一定要注意

   if(n<=0){            return false;        }

342. Power of Four

class Solution {public:bool isPowerOfFour(int n) {if (n <= 0){return false;}return (n&(n - 1)) == 0 && (n & 0x55555555)==n;}};

一方面是2的次幂，但是还有另外的特征，4的次幂中1处于奇数位上