Count 1 in Binary

Count how many 1 in binary representation of a 32-bit integer.

Have you met this question in a real interview? Yes
Example
Given 32, return 1

Given 5, return 2

Given 1023, return 9

Challenge
If the integer is n bits with m 1 bits. Can you do it in O(m) time?

public class Solution {
    /**
     * @param num: an integer
     * @return: an integer, the number of ones in num
     */
    public int countOnes(int num) {
        // write your code here
        int count =0;
        while(num!=0){
            num=num&(num-1);
            count++;
        }
        return count;

    }
};