Помогите написать код! С++ Строки (без использования классов)

 #include    

using namespace std; 

 

string multiple(const string& l, const char ch) 

{ 

    string result; 

    int add = 0; 

    int num; 

    for (auto i = l.crbegin(); i != l.crend(); i++) 

    { 

        num = (ch - '0') * (*i - '0') + add; 

        result.push_back(num % 10 + '0'); 

        add = num / 10; 

    } 

    if (add != 0) result.push_back(add + '0'); 

    reverse(result.begin(), result.end()); 

    return result; 

} 

 

string add(const string& l, const string& r) 

{ 

    string result; 

    bool left = true, right = true; 

    int num, add = 0; 

    auto l_it = l.crbegin(); 

    auto r_it = r.crbegin(); 

    do 

    { 

        num = (left?*l_it - '0':0) + (right?*r_it - '0':0) + add; 

        result.push_back(num % 10 + '0'); 

        add = num / 10; 

        if (left) 

        { 

            l_it++; 

            left = l_it != l.crend(); 

        } 

        if (right) 

        { 

            r_it++; 

            right = r_it != r.crend(); 

        } 

    } while (left || right); 

    if (add != 0) result.push_back(add + '0'); 

    reverse(result.begin(), result.end()); 

    return result; 

} 

 

int main()  

{ 

    bool negative{}; 

    string a, b, tmp, result = "0"; 

    cin >> a >> b; 

    if (a.length() < b.length()) swap(a, b); 

    if (a[0] == '-') 

    { 

        a = string(a.begin() + 1, a.end()); negative = !negative; 

    } 

    if (b[0] == '-') 

    { 

        b = string(b.begin() + 1, b.end()); negative = !negative; 

    } 

    int m10 = 0; 

    for (auto b_it = b.crbegin(); b_it != b.crend(); b_it++) 

    { 

        tmp = multiple(a, *b_it); 

        tmp.append(m10, '0'); 

        m10++; 

        result = add(result, tmp); 

    } 

    if (negative) result = '-' + result; 

    cout