Count 1 in Binary

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?

思路：
记住这个trick：每次n & (n – 1)，会把n去掉一个1。
例子：
1111 & 1110 = 1110 (4 -> 3)
1110 & 1101 ＝ 1100 (3 -> 2)

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;
    }
}