C++ Function to Check if Integer is Power of Two

Given an integer, write a function to determine if it is a power of two. This is not new, and the solution has been posted at Using a Faster Approach to Judge if a Integer is power of Two.

The solution is based on the fact that if the number if power of two, then do an logic-and operation with its minus-one will become zero.

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

Now, here is a bruteforce interesting solution:

class Solution {
public:
    int arr[50] = {
        1,
        2,
        4,
        8,
        16,
        32,
        64,
        128,
        256,
        512,
        1024,
        2048,
        4096,
        8192,
        16384,
        32768,
        65536,
        131072,
        262144,
        524288,
        1048576,
        2097152,
        4194304,
        8388608,
        16777216,
        33554432,
        67108864,
        134217728,
        268435456,
        536870912,
        1073741824
    };

    bool isPowerOfTwo(int n) {
        for (int i = 0; i < 31; i++) {
            if (arr[i] == n) {
                        return true;
            }
        }
        return false;
    }
};

and, similarly:

class Solution {
    private:
        unordered_set<int> power2{1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304, 8388608, 16777216, 33554432, 67108864, 134217728, 268435456, 536870912, 1073741824};
    public:
        bool isPowerOfTwo(int n) {
            return power2.find(n) != power2.end();
        }
    };

Or just the pure comparisons:

bool isPowerOfTwo(int n) {
    return n==1||n==2||n==4||n==8||n==16||n==32||n==64||n==128||n==256||n==512||n==1024||n==2048||n==4096||n==8192||n==16384||n==32768||n==65536||n==131072||n==262144||n==524288||n==1048576||n==2097152||n==4194304||n==8388608||n==16777216||n==33554432||n==67108864||n==134217728||n==268435456||n==536870912||n==1073741824;
}

And the following is based on recursion:

class Solution {
public:
    bool isPowerOfTwo(int n) {
        return n == 0 ? false : (n == 1 ? true : (n % 2 == 1 ? false : isPowerOfTwo(n / 2)));
    }
};

Do you have other fun solutions? Share below!

See also: Teaching Kids Programming – Algorithms of Power of Two and Check Integer is the Power of Two

–EOF (The Ultimate Computing & Technology Blog) —