HDU 1269

很水的TARJAN求强联通图的问题。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
const int N=10010;
const int M=100010;
int head[N],dfn[N],low[N],stack[N],st;
struct Edge{
	int u,v;
	int nxt;
}edge[M];
int tot,n,m,nTime,btn;
int vis[N];
bool instack[N];
void addedge(int u,int v){
	edge[tot].u=u;
	edge[tot].v=v;
	edge[tot].nxt=head[u];
	head[u]=tot++;
}

void tarjan(int u,int ufa){
	dfn[u]=low[u]=++nTime;
	stack[++st]=u;
	instack[u]=true;
	for(int e=head[u];e!=-1;e=edge[e].nxt){
		int v=edge[e].v;
	//	cout<<v<<endl;
		if(dfn[v]==-1){
			tarjan(v,u);
			low[u]=min(low[u],low[v]);
		}
		else if(instack[v])
		low[u]=min(low[u],dfn[v]);
	}
	if(dfn[u]==low[u]){
		int v; btn++;
		do{
			v=stack[st--];
			vis[v]=btn;
		}while(u!=v);
	}
}

int main(){
	int u,v;
	while(scanf("%d%d",&n,&m),n||m){
		memset(head,-1,sizeof(int)*(n+5));
		memset(dfn,-1,sizeof(int)*(n+5));
		memset(low,-1,sizeof(int)*(n+5));
		memset(vis,0,sizeof(int)*(n+5));
		memset(instack,false,sizeof(bool)*(n+5));
		tot=0;
		nTime=0;
		st=0;
		btn=0;
		for(int i=1;i<=m;i++){
			scanf("%d%d",&u,&v);
			addedge(u,v);
		}
		tarjan(1,0);
		bool flag=true;
		for(int i=2;i<=n;i++)
		if(vis[i]!=vis[1]){
			flag=false;
			break;
		}
		if(flag){
			puts("Yes");
		}
		else puts("No");
	}
	return 0;
}