Power of Two

Given an integer, write a function to determine if it is a power of two.

method:

1. recursive:

if x % 2 != 0, return false;

if x == 0, return false;

if x == 1, return true;

else, return isPowerOfTwo(x / 2);

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

 

2. trick

if x is power of 2, there is only one '1' in x(bit format). Using x & (x - 1) calculate the count of 1 in x.

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