hdu1269 迷宫城堡 (强连通分量Tarjan算法)

#include <stdio.h>
#include <string.h>
#define MAXN 10001

struct node
{
int from,to,next;
}edge[10*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));
}

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

void Tarjan(int x)
{
int i,u,temp;
dfn[x]=low[x]=++Dindex;//时间戳
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
{
temp=stack[--top];
instack[temp]=0;
} while (x!=temp);
}
}

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;
}

int main()
{
int n,m,i,x,y;
while(scanf("%d %d",&n,&m)&&(n+m)!=0)
{
Init();
while(m--)
{
//scanf("%d %d",&x,&y);
x=Scan();
y=Scan();
addEdge(x,y);
}
for(i=1;i<=n;++i)
{
if(!dfn[i])
Tarjan(i);
}
if(Bcnt==1)
printf("Yes\n");
else
printf("No\n");
}
return 0;
}