15. 二进制中 1 的个数

 public static int numberOf1(int n) {
        int count = 0;
        while(n != 0) {
            count ++;
            n = n&(n-1);
        }
        return count;
    }

    public static void main(String[] args) {
        System.out.println(numberOf1(4));
    }