HDU 1269 迷宫城堡 （有向图强连通分量Tarjan）

#include<cstdio>
#include<cstring>
#include<iostream>
#include<stack>
#define NODENUM 10005
#define EDGENUM 100005
using namespace std;

int N,M;

struct edgenode
{
int to,next;
}Edge[EDGENUM];
int head[NODENUM],edgenum;

bool in[NODENUM];
stack<int> S;
int rn,dfn[NODENUM],low[NODENUM],index;

void init()
{
memset(head,-1,sizeof(head));
memset(in,0,sizeof(in));
memset(dfn,0,sizeof(dfn));
index=rn=edgenum=0;
while(!S.empty()) S.pop();
}

void add(int a,int b)
{
++edgenum;
Edge[edgenum].next = head[a];
Edge[edgenum].to = b;
head[a] = edgenum;
}

void build()
{
while(M--)
{
int a,b;
scanf("%d %d",&a,&b);
add(a,b);
}
}

void tarjan(int u)
{
dfn[u] = low[u] = ++index;
S.push(u); in[u]=1;

for(int p=head[u];~p;p = Edge[p].next)
{
int i = Edge[p].to;
if(!dfn[i])
{
tarjan(i);
low[u] = min(low[u],low[i]);
}
else if(in[i]) low[u] = min(low[u],low[i]);
}

if(dfn[u] == low[u])
{
++rn;
int v;
do
{
v = S.top();
S.pop();
in[v]=0;
}
while(u!=v);
}
}

void work()
{
for(int i=1;i<=N;++i)
if(!dfn[i]) tarjan(i);
printf("%s\n",rn == 1? "Yes":"No");
}

int main()
{
while(~scanf("%d %d",&N,&M) && N+M)
{
init();
build();
work();
}
return 0;
}