POJ2186

  1 //poj2186 popular cows
  2 #include<cstdio>
  3 #include<iostream>
  4 #include<cstring>
  5 #include<algorithm>
  6 #include<stack>
  7 
  8 using namespace std;
  9 const int maxn=10010;
 10 const int maxm=50010;
 11 int n,m;
 12 struct edge
 13 {
 14     int u,v,next;
 15 }e[maxm],ee[maxm];
 16 int head[maxn],js,headd[maxn],jss;
 17 int dfsn[maxn],low[maxn],belong[maxn];
 18 bool ins[maxn];
 19 int sshu;
 20 int chudu[maxn],ss[maxn];
 21 stack<int>st;
 22 int visx;
 23 void init()
 24 {
 25     memset(head,0,sizeof(head));
 26     memset(e,0,sizeof(e));
 27     js=0;
 28     memset(headd,0,sizeof(headd));
 29     memset(ee,0,sizeof(ee));
 30     jss=0;
 31     memset(dfsn,-1,sizeof(dfsn));
 32     memset(low,-1,sizeof(low));
 33     sshu=0;
 34     memset(ss,0,sizeof(ss));
 35     while(!st.empty())st.pop();
 36     visx=0;
 37     memset(ins,0,sizeof(ins));
 38     memset(chudu,0,sizeof(chudu));
 39     memset(belong,0,sizeof(belong));
 40 }
 41 void addage(int u,int v,edge e[],int head[],int &js)
 42 {
 43     e[++js].u=u;e[js].v=v;
 44     e[js].next=head[u];head[u]=js;
 45 }
 46 
 47 void tarjan(int u)
 48 {
 49     dfsn[u]=low[u]=++visx;
 50     st.push(u);
 51     ins[u]=1;
 52     for(int i=head[u];i;i=e[i].next)
 53     {
 54         int v=e[i].v;
 55         if(dfsn[v]==-1)
 56         {
 57             tarjan(v);
 58             low[u]=min(low[u],low[v]);
 59         }
 60         else if(ins[v] && low[u]>dfsn[v])low[u]=dfsn[v];
 61     }
 62     if(low[u]==dfsn[u])
 63     {
 64         sshu++;
 65         int tp;
 66         do
 67         {
 68             tp=st.top();
 69             st.pop();
 70             ins[tp]=0;
 71             ss[sshu]++;
 72             belong[tp]=sshu;
 73         }while(u!=tp);
 74     }
 75 }
 76 int main()
 77 {
 78     while(scanf("%d%d",&n,&m)==2)
 79     {
 80         init();
 81         for(int i=0,u,v;i<m;i++)
 82         {
 83             scanf("%d%d",&u,&v);
 84             addage(u,v,e,head,js);
 85         }
 86         for(int i=1;i<=n;i++)
 87             if(dfsn[i]==-1)tarjan(i);
 88         for(int i=1;i<=m;i++)
 89         {
 90             int u=e[i].u,v=e[i].v;
 91             if(belong[u]!=belong[v])
 92             {
 93                 addage(belong[u],belong[v],ee,headd,jss);
 94                 chudu[belong[u]]++;
 95             }                
 96         }
 97         int count=0,mn;
 98         for(int i=1;i<=sshu;i++)
 99             if(chudu[i]==0)count++,mn=i;
100         if(count==1)printf("%d\n",ss[mn]);
101         else printf("0\n");
102     }
103     return 0;
104 }