A program to find factorial of 10000 in c++

What you want to use is a Big number class or library. Search on google to find one. For example:
https://mattmccutchen.net/bigint/[^]

I have been using the Number Theory Library[^] for dealing with large integers. It worked well for my needs.

AFAIK, Not a defined datatype - you would need to develop some own method. unsigned __int64 data type can store large values, but it's still not enough here.

Similar discussion here[^] with some possible ways.

#include<stdio.h>
#include<iostream>
#include<algorithm>
#define forr(i,a,b) for(int i=(a);i<(b);++i)
#define forn(i,n) forr(i,0,n)
#define dforn(i,n) for(int i=n-1;i>=0;--i)
#define forall(it,v) for(auto it=v.begin();it!=v.end();++it)
#define fst first
#define snd second
#define sz(c) ((int)c.size())
#define MAXP 1000000
#define BASE 1000000
#define LMAX 10000
using namespace std;
typedef long long ll;


struct bint {
	int l;//LONGITUD
	ll n[LMAX];//EL NUMERO 
	bint(ll x = 0) {
		l = 1;
		forn(i, LMAX) {
			if (x) l = i + 1;
			n[i] = x % BASE;
			x /= BASE;

		}
	}
	void out() {
		cout << n[l - 1];
		dforn(i, l - 1) printf("%6.6llu", n[i]);
	}
	void invar() {
		fill(n + l, n + LMAX, 0);
		while (l > 1 && !n[l - 1]) l--;
	}
};
bint operator*(const bint & a, ll b) {
	bint c;
	ll q = 0;
	forn(i, a.l) q += a.n[i] * b, c.n[i] = q % BASE, q /= BASE;
	c.l = a.l;
	while (q) c.n[c.l++] = q % BASE, q /= BASE;
	c.invar();
	return c;
}
int main() {
	int n = 10000;
	bint res(1);
	for (int i = 1; i <= n; ++i) {
		res = res * i;
	}
	res.out();
}

Ahi esta Campeón, en programación competitiva este tipo de cosas son pan de cada día