for循环

#include<iostream>

using namespace std;

int fact(int n)
{
    int res = 1;
    for(int i = 1;i <= n;i ++) res *= i;

    return res;
}

int main()
{
    int n;
    cin >> n;

    cout << fact(n) << endl;

    return 0;
}

递归

#include<iostream>

using namespace std;

int fact(int n)
{
    if(n == 1) return 1;

    return n * fact(n - 1);
}

int main()
{
    int n;
    cin >> n;

    cout << fact(n) << endl;

    return 0;
}