大数相减 C语言

#include <stdio.h>
#include <string.h>
using namespace std;
char a[1005],b[1005];
void Sub()
{
    int flag = 0;
    if(a == NULL || b == NULL)
        return;
    int len1 = strlen(a);
    int len2 = strlen(b);
    if(len1<len2)
    {
        int t = len1;
        len1 = len2;
        len2 = t;
        char c[1005];
        strcpy(c,a);
        strcpy(a,b);
        strcpy(b,c);
        flag = 1;
    }
    else if((len1 == len2)&&(a[0] < b[0]))
    {
        int t = len1;
        len1 = len2;
        len2 = t;
        char c[1005];
        strcpy(c,a);
        strcpy(a,b);
        strcpy(b,c);
        flag = 1;
    }

    int i, j;
    int num1[1005];
    int num2[1005];
    for (i = 0; i < len1; i++)
    {
        num1[i] = num2[i] = 0;
    }
    for (i = len1 - 1; i >= 0; i--)
    {
        num1[len1 - 1 - i] = a[i] - '0';
    }
    for (i = len2 - 1; i >= 0; i--)
    {
        num2[len2-1-i] = b[i] - '0';
    }
    for (i = 0; i < len1; i++)
    {
        num1[i] = num1[i] - num2[i];
        if(num1[i] < 0)
        {
            num1[i] = num1[i] + 10;
            num1[i+1] = num1[i+1] - 1;
        }
    }
    for (i = len1-1; i>=0 && num1[i] == 0; i--);
    if(flag == 1)
        printf("-");
    if(i >= 0)
        for (; i >= 0; i--)
        {
            printf("%d",num1[i]);
        }
    else
        printf("0");
}

int main(){
    scanf("%s%s",a,b);
    Sub();
    return 0;
}