大整数阶层N!

大整数阶层N!
写了一个版本，还可以优化，比如提取5、2等因子。

#include 
using namespace std;

const int DEMICAL = 10;

vector<int> multiply(vector<int>& a, int b)
{
    vector<int> c;
    int t = 0;
    for (int i = 0; i < a.size(); i++) {
        t += a[i] * b;
        c.push_back(t % DEMICAL);
        t /= DEMICAL;
    }
    while (t) {
        c.push_back(t % DEMICAL);
        t /= DEMICAL;
    }
    while (c.size() > 1 && c.back() == 0) {
        c.pop_back();
    }
    return c;
}

int main(void)
{
    FILE *stream;
    freopen_s(&stream, "./hoj19.1.input.txt", "r", stdin);
    int n;
    cin >> n;
    vector<int> a = vector<int>(1, 1);
    for (int i = 2; i <= n; i++) {
        a = multiply(a, i);
    }
    for_each(a.rbegin(), a.rend(), [](int num) { cout << to_string(num);  });
    return 0;
}