POJ2553 The Bottom of a Graph (Tarjan)

题目链接：http://poj.org/problem?id=2553

题解：A node v in a graph G=(V,E) is called a sink, if for every node w in G that is reachable from v, v is also reachable from w. The bottom of a graph is the subset of all nodes that are sinks, 强连通分量,出度为0,即为答案。

#include <stdio.h>      
#include <string.h>    
#define MAXN 5001    

struct node      
{      
	int to,next;      
}edge[100000];  

int n,belong[MAXN],outdegree[MAXN];      
int head[MAXN],instack[MAXN],low[MAXN],dfn[MAXN];        
int stack[MAXN],tot,Dindex,top,Bcnt;  

void Init()        
{//初始化        
	tot=0,top=0,Dindex=0,Bcnt=0;        
	memset(head,-1,sizeof(head));        
	memset(instack,0,sizeof(instack));        
	memset(dfn,0,sizeof(dfn));        
	memset(low,0,sizeof(low));       
	memset(belong,0,sizeof(belong)); 
	memset(outdegree,0,sizeof(outdegree));      
}  

int Scan()                
{                
	char ch;                
	int ret=0;                
	while((ch=getchar())<'0'||ch>'9');                
	while(ch>='0'&&ch<='9')                
	{                
		ret=ret*10+(ch-'0');                
		ch=getchar();                
	}                
	return ret;                
}   

void addEdge(int from,int to)        
{      
	edge[tot].to=to;        
	edge[tot].next=head[from];        
	head[from]=tot++;        
}   

void Tarjan(int x)  
{  
	int i,u,v;  
	dfn[x]=low[x]=++Dindex;  
	stack[top++]=x;  
	instack[x]=1;  
	for(i=head[x];i!=-1;i=edge[i].next)  
	{  
		u=edge[i].to;  
		if(!dfn[u])  
		{  
			Tarjan(u);  
			low[x]=low[x]>low[u]?low[u]:low[x];  
		}  
		else if(instack[u]&&low[x]>dfn[u])  
			low[x]=dfn[u];  
	}  
	if(low[x]==dfn[x])  
	{  
		Bcnt++;  
		do   
		{  
			v=stack[--top];  
			instack[v]=0;  
			belong[v]=Bcnt;  
		} while (x!=v);  
	}  
}  

int main()
{
	int n,e,v,u,i,j;
	while((n=Scan())&&n)
	{
		e=Scan();
		Init();
		while(e--)
		{
			v=Scan();
			u=Scan();
			addEdge(v,u);
		}
		for(i=1;i<=n;++i)
		{
			if(!dfn[i])
				Tarjan(i);
		}
		for(i=1;i<=n;++i)
		{
			for(j=head[i];j!=-1;j=edge[j].next)
			{
				v=edge[j].to;
				if(belong[i]!=belong[v])
					outdegree[belong[i]]++;
			}
		}
		v=0;
		for(i=1;i<=n;++i)
		{
			if(!outdegree[belong[i]])
			{
				if(!v)
				{
					printf("%d",i);
					v=1;
				}
				else
					printf(" %d",i);
			}
		}
		printf("\n");
	}
	return 0;
}