(tarjan求强连通)UOJ#146. 【NOIP2015】信息传递

UOJ#146. 【NOIP2015】信息传递

#include<iostream>
#include<cstdio>
#include<vector> 
#include<stack>
using namespace std;

const int maxn=2e5+7;
vector<int>E[maxn];
int dfn[maxn],low[maxn],tot,n,ans=maxn,vis[maxn];  //没必要开vis[],白书上可以直接用dfn[] 

stack<int>S;
void tarjan(int x){
	low[x]=dfn[x]=++tot;
	S.push(x);
	vis[x]=1;
	for(int i=0;i<E[x].size();i++){
		int v=E[x][i];
		if(!dfn[v]){
			tarjan(v);
			low[x]=min(low[x],low[v]); //树枝边 
		}else if(vis[v]){
			low[x]=min(low[x],dfn[v]); //横叉边 
		}
	}
	
	    if(low[x]==dfn[x]){
        int cnt = 0;
        while(1){
            int now = S.top();
            S.pop();
            vis[x]=0;
            cnt++;
            if(now==x)break;
        }
        if(cnt>1)ans=min(ans,cnt);
    }
}

int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        int x;
        scanf("%d",&x);
        E[i].push_back(x);
    }
    for(int i=1;i<=n;i++){
        if(!dfn[i])
            tarjan(i);
    }
    printf("%d\n",ans);
    return 0;
}