Count 1 in Binary详解编程语言

Count how many 1 in binary representation of a 32-bit integer. 
 
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) { 
        int count = 0; 
        while (num != 0) { 
            num = num & (num - 1); 
            count++; 
        } 
 
        return count; 
    } 
}