[LintCode] Count 1 in Binary

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?

 

Solution 1.  O(n) runtime

public class Solution {
    /**
     * @param num: an integer
     * @return: an integer, the number of ones in num
     */
    public int countOnes(int num) {
        int count = 0;
        for(int i = 0; i < Integer.SIZE; i++){
            if(((num >>> i) & 1) == 1){
                count++;    
            }
        }
        return count;
    }
}

 

Solution 2. O(m) runtime

public int countOnes(int num) {
    int count = 0;
    while(num != 0){
        num = num & (num - 1);
        count++;
    }
    return count;
}

 

 

Related Problems 

O(1) Check Power of 2