#include <iostream>
#include <vector>
// using namespace std;
using std::vector;
using std::ostream, std::operator<<;
class Number {
public:
vector <unsigned> self;
Number();
Number(int);
Number(vector <unsigned>);
Number operator*=(const Number&);
friend ostream& operator << (ostream& os, const Number&);
};
using std::string;
void reverse(vector <unsigned>&);
void swap(unsigned&, unsigned&);
vector <unsigned> int_to_vector(int);
vector <unsigned> int_to_vector(string);
Number::Number()
{
this->self = vector <unsigned>(1, 0);
}
Number::Number(int num)
{
this->self = int_to_vector(num);
}
Number::Number(vector<unsigned> num)
{
this->self = num;
}
Number Number::operator*=(const Number& number)
{
vector <unsigned>
A = this->self,
B = number.self,
C(A.size() + B.size(), 0);
unsigned a, b, c;
reverse(A), reverse(B);
for (int x = 0; x < B.size(); ++x) {
for (int y = 0; y < A.size(); ++y) {
a = A[y], b = B[x];
c = a * b;
C[x + y] += c;
}
}
for (int i = 0; i < C.size(); ++i) {
c = C[i];
if (c > 9) {
C[i + 1] += c / 10;
C[i] %= 10;
}
}
reverse(C);
this->self = C;
return *this;
}
using std::endl;
ostream& operator<<(ostream& os, const Number& number)
{
bool flag = false;
for (auto num : number.self) {
if (num) {
flag = true;
}
if (flag) {
os << num;
}
}
os << endl;
return os;
}
Number factorial(int n) {
Number number(1);
do {
number *= n;
n--;
} while (n);
return number;
}
using std::cout;
int main(void) {
int n = 999; // <------------------------------------------
cout << factorial(n);
return 0;
}
void reverse(vector <unsigned> & num) {
for (int i = 0; i < num.size() >> 1; ++i) {
swap(num[i], num[num.size() - i - 1]);
}
}
void swap(unsigned& A, unsigned& B) {
unsigned temp = A;
A = B;
B = temp;
}
vector<unsigned> int_to_vector(int num)
{
vector <unsigned> res;
do {
res.push_back(num % 10);
num /= 10;
} while (num);
reverse(res);
return res;
}
vector<unsigned> int_to_vector(string num)
{
vector <unsigned> res;
for (char c : num) {
res.push_back(c - '0');
}
return res;
}
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