HDU 1269 —— 迷宫城堡

#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
using namespace std;
const int maxn = 11000;
const int maxm = 110000;
int n, m;
int head[maxn], edgenum;
int DFN[maxn], Low[maxn], Stack[maxn], Belong[maxn];
bool Instack[maxn];
int Time, taj, top;

struct Edge
{
int from, to, next;
}edge[maxm];

void add(int u, int v)
{
edge[edgenum].from = u;
edge[edgenum].to = v;
edge[edgenum].next = head[u];
head[u] = edgenum++;
}

void init()
{
memset(head, -1, sizeof head);
edgenum = 0;
memset(DFN, -1, sizeof DFN);
memset(Instack, false, sizeof Instack);
Time = taj = top = 0;
}

void Tarjan(int u)
{
DFN[u] = Low[u] = ++Time;
Stack[top++] = u;
Instack[u] = true;
for(int i = head[u];i != -1;i = edge[i].next)
{
int v = edge[i].to;
if(DFN[v] == -1)
{
Tarjan(v);
Low[u] = min(Low[u], Low[v]);
}
else if(Instack[v] && Low[u] > DFN[v])
Low[u] = DFN[v];
}
if(DFN[u] == Low[u])
{
taj++;
while(1)
{
int now = Stack[--top];
Instack[now] = false;
Belong[now] = taj;
if(now == u)
break;
}
}
}

int main()
{
while(~scanf("%d%d", &n, &m))
{
if(n == 0 && m == 0)	break;
init();
while(m--)
{
int u, v;
scanf("%d%d", &u, &v);
add(u, v);
}
for(int i = 1;i<=n;i++)
{
if(DFN[i] == -1)
Tarjan(i);
}
if(taj == 1)	printf("Yes\n");
else	printf("No\n");
}
return 0;
}