UVa10219 - Find the ways !

#include <stdio.h>
#include <string.h>

#define MAX 11000

void reverse(char *from, char *to);
void call_mult(char *first, char *second, char *result);
void call_minus(char *first, char *second, char *result);
void call_add(char *first, char *second, char *result);
void call_divide(char *first, int div, char *result);

int main()
{
    char buf[MAX], temp[MAX];
    int n, k, i;
    char res[MAX];

    setvbuf(stdout, NULL, _IONBF, 0);

    while (scanf("%d%d", &n, &k) != EOF)
    {
        sprintf(buf, "%d", n);

        for (i = 1; i < k; i++)
        {
            sprintf(temp, "%d", n - i);
            call_mult(buf, temp, res);
            strcpy(buf, res);
        }


        for (i = 2; i <= k; i++)
        {
            call_divide(buf, i, res);
            strcpy(buf, res);

        }

        printf("%d\n", strlen(res));

    }


    return 0;
}


void reverse(char *from, char *to)
{
    int len = strlen(from), i;

    for (i = 0; i < len; i++)
    {
        to[i] = from[len - 1 - i];
    }
    to[len] = '\0';
}

void call_mult(char *first, char *second, char *result)
{
    char F[MAX], S[MAX], Res[MAX];
    int f_len, s_len, f, s, r, r_len, sum, carry;

    f_len = strlen(first);
    s_len = strlen(second);
    r_len = f_len + s_len;
    r = -1;
    reverse(first, F);
    reverse(second, S);

    for (f = 0; f < r_len; f++)
        Res[f] = '0';
    Res[f] = '\0';

    for (s = 0; s < s_len; s++)
    {
        for (f = 0, carry = 0; f < f_len; f++)
        {
            sum = (F[f] - '0') * (S[s] - '0') + (Res[f + s] - '0') + carry;
            Res[f + s] = sum % 10 + '0';
            carry = sum / 10;
            if (f + s > r)
                r = f + s;
        }

        while (carry)
        {
            sum = (Res[f + s] - '0') + carry;
            Res[f + s] = sum % 10 + '0';
            carry = sum / 10;
            if (f + s > r)
                r = f + s;
            f++;
        }
    }

    for (; r > 0 && Res[r] == '0'; r--);
    Res[r + 1] = '\0';
    reverse(Res, result);
}

void call_minus(char *first, char *second, char *result)
{
    char F[MAX], S[MAX];
    int f_len, s_len, now, borrow, sum;

    f_len = strlen(first);
    s_len = strlen(second);
    reverse(first, F);
    reverse(second, S);

    for (now = 0, borrow = 0; now < f_len && now < s_len; now++)
    {
        sum = (F[now] - '0') - (S[now] - '0') - borrow;
        if (sum < 0)
        {
            borrow = 1;
            F[now] = sum + 10 + '0';
        }
        else
        {
            borrow = 0;
            F[now] = sum + '0';
        }
    }

    for (; now < f_len && borrow; now++)
    {
        sum = (F[now] - '0') - borrow;
        if (sum < 0)
        {
            borrow = 1;
            F[now] = sum + 10 + '0';
        }
        else
        {
            borrow = 0;
            F[now] = sum + '0';
        }
    }

    for (now = f_len - 1; now > 0 && F[now] == '0'; now--);
    F[now + 1] = '\0';
    reverse(F, result);

}

void call_add(char *first, char *second, char *result)
{
    char F[MAX], S[MAX], Res[MAX];
    int f_len, s_len, now, carry, sum;

    f_len = strlen(first);
    s_len = strlen(second);
    reverse(first, F);
    reverse(second, S);

    for (now = 0, carry = 0; now < f_len && now < s_len; now++)
    {
        sum = (F[now] - '0') + (S[now] - '0') + carry;
        Res[now] = sum % 10 + '0';
        carry = sum / 10;
    }

    for (; now < f_len; now++)
    {
        sum = (F[now] - '0') + carry;
        Res[now] = sum % 10 + '0';
        carry = sum / 10;
    }

    for (; now < s_len; now++)
    {
        sum = (S[now] - '0') + carry;
        Res[now] = sum % 10 + '0';
        carry = sum / 10;
    }

    if (carry)
        Res[now++] = carry + '0';
    Res[now] = '\0';

    reverse(Res, result);
}


void call_divide(char *first, int div, char *result)
{
    char Res[MAX];
    int len = strlen(first);
    int remainder = 0, now, sum;

    for (now = 0; now < len; now++)
    {
        sum = remainder * 10 + (first[now] - '0');
        Res[now] = sum / div + '0';
        remainder = sum % div;
    }
    Res[now] = '\0';

    for (now = 0; Res[now] == '0'; now++);
    strcpy(result, &Res[now]);
    if (strlen(result) == 0)
        strcpy(result, "0");
}