1000!.c

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

#define long long long
#define nln printf("%c", '\n');

typedef char bignum[1000000];

void inra(bignum N)
{
    printf("%s\n", N);
}

void erase(bignum n, long x, long y) // erase bignum n, from index x, y character
{
    long l = strlen(n);
    for (int i = x+y; i < l; ++i)
        n[i-y] = n[i];
    for (int i = l-y; i < l; ++i)
        n[i] = 0;
}

void del_zero(bignum str)
{
    while (str[0] == '0' && strlen(str) > 1)
        erase(str, 0, 1);
}

void fill0(bignum N, long n)
{
    N = strrev(N);
    long l = strlen(N);
    for (long i = 1; i < n-l+1; ++i)
        N[l-1+i] = '0';
    N = strrev(N);
}

void reverse(char str[])
{
    long idx = strlen(str);
    for (long i = 0; i < idx/2; ++i){
        char tmp = str[i];
        str[i] = str[idx-i-1];
        str[idx-i-1] = tmp;
    }
}

const char* sum(bignum a, bignum b)
{
    if (strlen(a) < strlen(b))
        fill0(a, strlen(b));
    else
        fill0(b, strlen(a));

    char res[10000] = "";
    long rem = 0, idx = 0;
    for (long i = strlen(b)-1; i >= 0; --i){
        long msm = (a[i]-'0')+(b[i]-'0')+rem;
        char c = msm % 10 + '0';
        res[idx++] = c;
        rem = msm / 10;
    }

    if (rem != 0)
        res[idx++] = rem+'0';

    reverse(res);

    // convers to return -> I still don't know why have to do this
    char *rt = res;
    return rt;
}

const char* single_multi(bignum a, long b)
{
    char res[10000] = "";
    long rem = 0, idx = 0;
    for (long i = strlen(a)-1; i >= 0; --i){
        long mmt = (a[i]-'0')*b+rem;
        char c = mmt % 10 + '0';
        res[idx++] = c;
        rem = mmt / 10;
    }

    if (rem != 0)
        res[idx++] = rem+'0';

    reverse(res);

    char *rt = res;
    del_zero(rt);
    return rt;
}

const char* multi(bignum a, bignum b)
{
    char res[10000] = "";
    for (long i = strlen(b)-1; i >= 0; --i){
        char smt[10000] = ""; strcpy(smt, single_multi(a, b[i]-'0'));
        long add = strlen(b)-1-i;
        long l = strlen(smt);
        while (add--)
            smt[l] = '0', l += 1;
        char tmp[10000] = ""; strcpy(tmp, sum(res, smt));
        strcpy(res, tmp);
    }

    char *rt = res;
    return rt;
}

long fastpow(long a, long b)
{
    if (b == 0)
        return 1;
    long tmp = fastpow(a, b/2);
    if (b % 2 == 1)
        return tmp*tmp*a;
    else
        return tmp*tmp;
}

long bignum2num(bignum n)
{
    long res = 0;
    for (long i = 0; i < strlen(n); ++i)
        res += (n[i]-'0')*fastpow(10, strlen(n)-i-1);
    return res;
}

const char *num2bignum(long n)
{
    char res[10000] = "";
    long idx = 0;
    while (n > 0){
        char c = n % 10 + '0';
        res[idx++] = c;
        n /= 10;
    }

    reverse(res);

    char *rt = res;
    return rt;
}



const char *factorial(long n)
{
    char res[10000] = "1";
    n++;
    while (n--)
        if (n > 0){
            char N[10000]; strcpy(N, num2bignum(n));
            char tmp[10000]; strcpy(tmp, multi(res, N));
            strcpy(res, tmp);
        }

    char *rt = res;
    return rt;
}

int main()
{
    printf("Tinh giai thua cua: ");
    long n;
    scanf("%lli", &n);
    printf("%s", factorial(n));
    return 0;
}