在dfs函数运行过程中，k的取值会满足1≤k≤n+1。

#include <algorithm>
#include <cstdio>
#include <cstring>
bool flag[27];
int n;
int p[27];
int ans = 0;

void dfs(int k) {
    if (k == n + 1) {
        ++ans;
        return;
    }
    for (int i = 1; i <= n; ++i) {
        if (flag[i]) continue;
        if (k > 1 && i == p[k - 1] + 1) continue;
        p[k] = i;
        flag[i] = true;
        dfs(k + 1);
        flag[i] = false;
    }
    return;
}

int main() {
    scanf("%d", &n);
    dfs(1);
    printf("%d\n", ans);
    return 0;
}