为什么解决方案是这样的?

从Java马利克教科书-确定一个数字是否可被11整除。

提供的代码解决方案:

import java.util.*;

public class Divby11
{
    static Scanner console = new Scanner(System.in);

    public static void main (String[] args)
    {
        int num, temp, sum;
        char sign;

        System.out.print("Enter a positive integer: ");
        num = console.nextInt();
        System.out.println();

        temp = num;

        sum = 0;
        sign = '+';

        do
        {
            switch (sign)
            {
            case '+' :
                sum = sum + num % 10;
                sign = '-';
                break;

            case '-' :
                sum = sum - num % 10;
                sign = '+';
            }

            num = num / 10;       //remove the last digit
        }
        while (num > 0);

        if (sum % 11 == 0)
            System.out.println(temp + " is divisible by 11");
        else
            System.out.println(temp + " is not divisible by 11");
    }

为什么要经过以上所有的努力,然后说…

  if (sum % 11 == 0)
            System.out.println(temp + " is divisible by 11");
        else
            System.out.println(temp + " is not divisible by 11");

你们中的任何一个专家都能明白为什么作者会这样做(很长的路)?