1.暴力代码 2/10

#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;
}

 2.使用了组合数学里的错排公式

错排公式：n个编号和n个位置全部不对应。M(n)=(n-1)[M(n-2)+M(n-1)]

M(1)=0，M(2)=1。

所以在写代码的时候，a[0]=0,a[1]=0,a[2]=1.

注意点：错排公式，开long long

#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;
}