https://www.luogu.org/problemnew/show/P1444

题解：

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q;
int ans,cnt,flag,temp,sum;
int a[N];
char str;
struct node {
    int x,y;
    bool operator <(const node &S)const{
        return y<S.y || (y==S.y && x<S.x);
    }
} p[N];
int to[N];
bool tag[N];
int con[N];
bool cycle(int x) {
    while(to[x]) {
        if(tag[x]) return 1;
        tag[x]=1;
        x=con[to[x]];
    }
    return 0;
}
void dfs1(int k) {
    if(k>n) {
        bool ok=0;
        for(int i=1;i<=n && !ok;i++)
            memset(tag,0,sizeof(tag)), ok|=cycle(i);
        ans+=ok;
        return;
    }
    if(con[k]) dfs1(k+1);
    else {
        for(int i=k+1;i<=n;i++)
            if(!con[i]) {
                con[i]=k, con[k]=i;
                dfs1(k+1);
                con[i]=con[k]=0;
            }
    }
}
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    //scanf("%d",&t);
    //while(t--){
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%d%d",&p[i].x,&p[i].y);
    sort(p+1,p+1+n);
    for(int i=1;i<=n-1;i++)
        if(p[i].y==p[i+1].y) to[i]=i+1;
    dfs1(1);
    printf("%d\n",ans);
    //}

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}