如何在该代码基础上加入负数运算

问题遇到的现象和发生背景

如何在源代码基础上加入负数运算

问题相关代码，请勿粘贴截图

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h>
#include<iostream>
#include<string.h>
#include<stdlib.h>
using namespace std;
#define maxn 10
#define N 100005
int a[10000000];
int pp = 0;

struct Sqlist
{
    int data;
    struct Sqlist* next, * before;
}; 
struct Sqlist* r1, * str1, * s1, * s11, * r2, * str2, * s2, * s22;

void  ab()
{
    char s[N], ss1[N], ss2[N];
    int len1, len2, ans = 0, flag = 0, i;
    scanf("%s%s", ss1, ss2);
    len1 = strlen(ss1);
    len2 = strlen(ss2);
    r1 = (struct Sqlist*)malloc(sizeof(struct Sqlist));
    r1->before = NULL;
    r2 = (struct Sqlist*)malloc(sizeof(struct Sqlist));
    r2->before = NULL;
    str1 = r1;
    for (i = len1 - 1; i >= 0; --i)
    {
        s1 = (struct Sqlist*)malloc(sizeof(struct Sqlist));
        s1->data = ss1[i] - '0';
        s1->next = NULL;
        s1->before = str1;
        str1->next = s1;
        str1 = str1->next;
    }
    s1 = (struct Sqlist*)malloc(sizeof(struct Sqlist) );
    s1->data = 0;
    s1->next = NULL;
    s1->before = str1;  
    str1->next = s1;
    str1 = str1->next;

    str2 = r2;
    for (i = len2 - 1; i >= 0; --i)
    {
        s2 = (struct Sqlist*)malloc(sizeof(struct Sqlist));
        s2->data = ss2[i] - '0';
        s2->next = NULL;
        s2->before = str2;
        str2->next = s2;
        str2 = str2->next;
    }
    
    for (i = 0; i <= len1 - len2; ++i)
    {
        s2 = (struct Sqlist*)malloc(sizeof(struct Sqlist));
        s2->data = 0;
        s2->next = NULL;
        s2->before = str2;
        str2->next = s2;
        str2 = str2->next;
    }
    ans = 0;
    str1 = r1->next;
    str2 = r2->next;
    for (i = 0; i <= len1; ++i)
    {

        ans = ans + str1->data + str2->data;
        str1->data = ans % 10;
        ans /= 10;
        str1 = str1->next;
        str2 = str2->next;
    }

    flag = 0;
    ans = 0;
    str1 = r1->next;
    while (1)
    {
        if (str1->next == NULL) break;
        str1 = str1->next;
    }
    while (str1 != NULL)
    {
        if (str1->data != 0)
            flag = 1;
        if (flag)
        {
            ans++;
            a[pp] = str1->data;
            pp++;
        }
        str1 = str1->before;
        if (str1->before == NULL) break;
    }
    printf("\n");
}

int main()
{
    memset(a, 0, sizeof(a));  
    cout << "请输入数据：" << endl;
    ab();
    int xx = pp % 4;
    int pan = 0;
    cout << "输出结果为：" << endl;
    for (int i = 0; i < pp - 4; i++)
    {
        cout << a[i];
        pan++;
        if (i == xx - 1)  
        {
            cout << ",";
            pan = 0;
        }
        if (pan % 4 == 0 && pan != 0) cout << ",";  //四位一输出 
    }
    for (int i = pp - 4; i < pp; i++)
        cout << a[i];
        cout << endl;
    return 0;
}