[LintCode] O(1) Check Power of 2

Using O(1) time to check whether an integer n is a power of 2.

Example

For n=4, return true;

For n=5, return false;

Challenge 

O(1) time

 

Solution 1.

class Solution {
    public boolean checkPowerOf2(int n) {
        if(n <= 0) {
            return false;
        }
        while(n != 1) {
            if(n % 2 != 0) {
                return false;
            }
            n = n / 2;
        }
        return true;
    }
}

 

Solution 2.  Bitwise operation

If n is a power of 2, then n's binary representation must have and only have one 1. So this problem is equivalent with Count 1 in Binary.

class Solution {
    public boolean checkPowerOf2(int n) {
        if(n <= 0){
            return false;
        }
        int cnt = 0;
        while(n != 0){
            if((n & 1) == 1){
                cnt++;
                if(cnt > 1){
                    return false;
                }
            }
            n = n >>> 1;
        }
        return true;
    }
}

 

Related Problems 

Count 1 in Binary