Power of Two

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

Analyse: ***Notice that if a number is a power of 2, then the highest binary digit is 1 and the remaining digits are all 0s. So n & (n-1) = 0. If a number is not a power of 2, then n & (n - 1) != 0.

Runtime: 4ms.

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

 

Other solution:

1. Runtime: 8ms.

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

2. Rutime: 8ms

class Solution {
public:
    bool isPowerOfTwo(int n) {
        if(n <= 0 ) return false;
        
        long long num = 1;
        while(num <= n){
            if(num == n) return true;
            num <<= 1;
        }
        return false;
    }
};