PowerOfFour.cpp

// Source : https://leetcode.com/problems/power-of-four/
// Author : Hao Chen
// Date   : 2016-05-29

/*************************************************************************************** 
 *
 * 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 @yukuairoy  for adding this problem and creating all test 
 * cases.
 ***************************************************************************************/


class Solution {
public:
    bool isPowerOfFour(int num) {
        static int mask = 0b01010101010101010101010101010101;
        
        //edge case
        if (num<=0) return false;
        
        // there are multiple bits are 1
        if ((num & num-1) != 0) return false;
        
        // check which one bit is zero, if the place is 1 or 3 or 5 or 7 or 9...,
        // then it is the power of 4
        if ((num & mask) != 0) return true;
        return false;
    }
};