POJ——T2186 Popular Cows || 洛谷——P2341 [HAOI2006]受欢迎的牛

 1 #include <algorithm>
 2 #include <cstring>
 3 #include <cstdio>
 4 
 5 using namespace std;
 6 
 7 const int N(50015);
 8 int n,m,u,v,sumchudu,ans,cnt;
 9 int point[N],chudu[N];
10 int sumedge,head[N];
11 struct Edge
12 {
13     int from,to,next;
14     Edge(int from=0,int to=0,int next=0) :
15     from(from),to(to),next(next) {}
16 }edge[N];
17 
18 int ins(int from,int to)
19 {
20     edge[++sumedge]=Edge(from,to,head[from]);
21     return head[from]=sumedge;
22 }
23 
24 int dfn[N],low[N],tim;
25 int Stack[N],top,instack[N];
26 int col[N],sumcol;
27 
28 void DFS(int now)
29 {
30     dfn[now]=low[now]= ++tim;
31     Stack[++top]=now; instack[now]=1;
32     for(int i=head[now];i;i=edge[i].next)
33     {
34         v=edge[i].to;
35         if(instack[v]) low[now]=min(low[now],dfn[v]);
36         else if(!dfn[v])
37               DFS(v), low[now]=min(low[now],low[v]);
38     }
39     if(low[now]==dfn[now])
40     {
41         col[now]= ++sumcol;
42         point[sumcol]++;
43         for(;Stack[top]!=now;top--)
44         {
45             point[sumcol]++;
46             col[Stack[top]]=sumcol;
47             instack[Stack[top]]=0;
48         }
49         instack[now]=0; top--;
50     }
51 }
52 
53 int main()
54 {
55     scanf("%d%d",&n,&m);
56     for(int i=1;i<=m;i++)
57         scanf("%d%d",&u,&v),ins(u,v);
58     for(int i=1;i<=n;i++)
59         if(!dfn[i]) DFS(i);
60     for(int i=1;i<=m;i++)
61     {
62         u=edge[i].from; v=edge[i].to;
63         if(col[u]!=col[v]) chudu[col[u]]++;
64     }
65     for(int i=1;i<=sumcol;i++) if(!chudu[i])
66         ++sumchudu,ans=point[i];
67     if(sumchudu!=1) ans=0;
68     printf("%d\n",ans);
69     return 0;
70 }