Magic necklace （项链排列）

题目链接：https://ac.nowcoder.com/acm/contest/3570/H

 

 

 

 

 

 

 

#include <iostream>
#include <algorithm>
#include <string>
#include <string.h>
#include <vector>
#include <map>
#include <stack>
#include <set>
#include <queue>
#include <time.h>

#define LL long long
#define INF 0x3f3f3f3f
#define ls nod<<1
#define rs (nod<<1)+1

const int maxn = 1e5 + 10;
const LL MOD = 1e9 + 7;

template<class T>inline void read(T &res)
{
    char c;T flag=1;
    while((c=getchar())<'0'||c>'9')if(c=='-')flag=-1;res=c-'0';
    while((c=getchar())>='0'&&c<='9')res=res*10+c-'0';res*=flag;
}

int ans[20],vis[20],a[20];
int n,cnt;

void dfs(int k) {
    if (k == n+1) {
        if (abs(a[1]-a[n]) != 1)
            cnt++;
        return ;
    }
    for (int i = 1;i <= n;i++) {
        if (!vis[i] && (abs(i-a[k-1]) != 1 || k == 1)) {
            a[k] = i;
            vis[i] = 1;
            dfs(k+1);
            a[k] = 0;
            vis[i] = 0;
        }
    }
}

int main() {
    for (n = 1;n <= 11;n++) {
        memset(a,0, sizeof(a));
        memset(vis,0, sizeof(vis));
        cnt = 0;
        dfs(1);
        ans[n] = cnt / n / 2;
    }
    ans[1] = 1;
    int t;
    scanf("%d",&t);
    while (t--) {
        scanf("%d",&n);
        printf("%d\n",ans[n]);
    }
    return 0;
}