【11.13 测试】Tourist Attractions

 

题解：考场上打得一手好暴力。DFS骗得40分

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdlib>
#include<queue>
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1502;
const int M=1125752;
int n;
char s[N];
struct node{
    int to;
    int next;
}e[M*2];
int head[N],cnt,vis[N];
void add(int x,int y){
    e[++cnt].to=y;
    e[cnt].next=head[x];
    head[x]=cnt;
}
ll ans;
void dfs(int now,int sum){
    if(sum==4) { ans++; return; }
    for(int i=head[now];i;i=e[i].next){
        int v=e[i].to;
        if(vis[v]==1) continue;
        vis[v]=1; dfs(v,sum+1); vis[v]=0;
    }
}
int main(){
    freopen("tour.in","r",stdin);
    freopen("tour.out","w",stdout);
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%s\n",s+1);
        for(int j=1;j<=n;j++)
            if(s[j]=='1')  add(i,j);        
    }
    for(int i=1;i<=n;i++) 
        { vis[i]=1; dfs(i,1); vis[i]=0; }
    printf("%lld",ans);
    return 0;
}