【tarjan】BZOJ 1051:受欢迎的牛

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<cmath>
 5 
 6 #define maxn 10001
 7 
 8 using namespace std;
 9 
10 inline int In()
11 {
12     int x=0;char ch=getchar();
13     while(ch<‘0‘||ch>‘9‘)ch=getchar();
14     while(ch>=‘0‘&&ch<=‘9‘)x=x*10+ch-‘0‘,ch=getchar();
15     return x;
16 }
17 
18 struct ed{
19     int u,v;
20 }edge[maxn*5];
21 
22 struct node{
23     int to,last;
24 }e[maxn*10];
25 
26 int last[maxn],tot=0,dfn[maxn],low[maxn],father[maxn],sta[maxn],top=0,cnt=0,size=0,num[maxn],in[maxn];
27 
28 bool ins[maxn];
29 
30 void add(int u,int v){e[++tot].to=v,e[tot].last=last[u],last[u]=tot;}
31 
32 void tarjan(int poi)
33 {
34     dfn[poi]=low[poi]=++cnt;
35     ins[poi]=1;sta[++top]=poi;
36     for(int i=last[poi];i;i=e[i].last)
37     {
38         int u=e[i].to;
39         if(!dfn[u])
40         {
41             tarjan(u);
42             low[poi]=min(low[u],low[poi]);
43         }
44         else if(ins[u]) low[poi]=min(low[poi],dfn[u]);
45     }
46     if(dfn[poi]==low[poi])
47     {
48         size++;
49         int vv;
50         do{
51             vv=sta[top];
52             father[vv]=size;
53             ins[vv]=0;
54             num[size]++;
55             top--;
56         }while(vv!=poi);
57     }
58 }
59 
60 int main()
61 {
62     freopen("1051.in","r",stdin);
63     int n,m,str=0;
64     n=In(),m=In();
65     for(int i=1;i<=n;i++)father[i]=i;
66     for(int i=1;i<=m;i++)
67         edge[i].u=In(),edge[i].v=In(),add(edge[i].u,edge[i].v);
68     for(int i=1;i<=n;i++)if(!dfn[i])tarjan(i);
69     memset(last,0,sizeof(last));
70     tot=0;
71     for(int i=1;i<=m;i++)if(father[edge[i].u]!=father[edge[i].v]){
72         in[father[edge[i].u]]++;
73     }
74     for(int i=1;i<=size;i++)
75     {
76         if(!in[i]&&str){printf("0");return 0;}
77         if(!in[i])str=i;
78     }
79     printf("%d",num[str]);
80     return 0;
81 }