MT2046 巨大的错误

#include <bits/stdc++.h>
using namespace std;
int n;
int a[25];
int b[25];
int ans = 0;
bool err()
{
    for (int i = 1; i <= n; i++)
    {
        if (a[i] == b[i])
        {
            return false;
        }
    }
    return true;
}
int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        a[i] = i;
        b[i] = i;
    }
    do
    {
        do
        {
            if (err())
            {
                ans++;
            }
        } while (next_permutation(b + 1, b + n + 1));
    } while (next_permutation(a + 1, a + n + 1)); // 求全排列
    cout << ans / n;
}

#include <bits/stdc++.h>
using namespace std;
long long int n;
long long int a[25];
int main()
{
    cin >> n;
    a[0] = 0;
    a[1] = 0;
    a[2] = 1;
    for (long long int i = 3; i <= n; i++)
    {
        a[i] = (i - 1) * (a[i - 2] + a[i - 1]);
    }
    cout << a[n];
    return 0;
}