正确实现大整数的减法运算符

Question

我正在编写一个程序，该程序处理存储在列表中的大整数，并能够通过重载执行比较和运算符。我在使减法运算符重载时遇到麻烦，程序可以编译，但是运算会导致错误的结果。

操作员部分：

BigInt BigInt::operator-(BigInt addend2) {
    BigInt total;
    short int first, second, result;

    list<short int>::reverse_iterator
        it1 = myList.rbegin(),
        it2 = addend2.myList.rbegin();

    while (it1 != myList.rend() || it2 != addend2.myList.rend()) {
        if (it1 != myList.rend()) { first = *it1; }
        else { first = 0; }
        if (it2 != addend2.myList.rend()) { second = *it2; }
        else { second = 0; }


        if (first < second) {
            first = first + 1000;
            short int temp = first - second;
            result = temp % 1000;
            first = first - 1000;
        }
        else {
            long int temp = first - second;
            result = temp % 1000;
        }

        if (it1 != myList.rend()) {
            it1++;
            if (first < second) {
                first = *it1;
                first = first - 1;
                *it1 = first;
            }
        }

        if (it2 != addend2.myList.rend()) { it2++; }
        total.myList.push_front(result);
    }
    return total;
}

当我在以下测试文件中运行时：

cout << "\nThe subtraction of\n\t"
           << number1 << " - " << number2
           << "\nis\n\t" << number1 - number2 << endl;

我收到如下输出：

Enter a big integer:
50000
Enter another big integer:
20800
The sum of
        50000
 + 20800

is
        70800


The bigger number of
        50000

and
        20800

is
        50000


The subtraction of
        4-1000
 - 20800

is
        299999200


Add more integers (Y or N)?

为什么减法运算符会打印出意外的结果？