大数相减 C语言

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

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

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