题目链接

#include <iostream>
#include <cstring>
#include <unordered_map>
using namespace std;

int mp[15][15];
int n, m;
int ans;
bool st[15];
int res[15];

void dfs(int u){
    if(u == n+1){
        if(mp[1][res[n]] || mp[res[n]][1])return;
        ans ++;
        return;
    }
    for(int i = 1; i <= n; i++){
        if(st[i]) continue;
        if(mp[res[u-1]][i] || mp[i][res[u-1]]) continue;
        st[i] = 1;
        res[u] = i;
        dfs(u+1);
        st[i] = 0;
    }

}

int main(){
    memset(res, 0, sizeof res);
    memset(st, 0, sizeof st);
    cin >> n >> m;
    for(int i = 1; i <= m; i++){
        int a, b;
        cin >> a >> b;
        mp[a][b] = 1;
        mp[b][a] = 1;
    }
    st[1] = 1;
    res[1] = 1;
    dfs(2);
    cout << ans;
    return 0;
}