#define max(a, b) ((a) > (b) ? (a) : (b))

static char* reverse(char *str)

{

    char *l, *r, c;

    for (l = str, r = str + strlen(str) - ; l < r; l++, r--) {

        c = *l;

        *l = *r;

        *r = c;

    }

    return str;

}

/*

 * Return the result of a big int as string when succeeded; otherwise

 * a NULL is returned. The caller needs to take care of the returned

 * heap memory; caller needs to ensure the inputs are valid big int.

 */

char* addBigInt(char *i1, char *i2)

{

    char *p1, *p2, *p3, *res;

    int len1, len2, val, carry;

    if (!i1 || !*i1)

        return strdup(i2);

    if (!i2 || !*i2)

        return strdup(i1);

    len1 = strlen(i1);

    len2 = strlen(i2);

    if ((res = (char*)malloc(max(len1, len2) + )) == NULL)

        return NULL;

    for (p1 = i1 + len1 - , p2 = i2 + len2 - , p3 = res, val = ;

        p1 != i1 -  || p2 != i2 -  || val; 
　　　　 *p3++ = val %  + '', val /= ) {

        if (p1 != i1 - )

            val += *(p1--) - '';

        if (p2 != i2 - )

            val += *(p2--) - '';

    }

    *p3 = '\0';

    return reverse(res);

}