POJ 2186 Popular Cows

 1 #include <cstdio>
 2 #include <iostream>
 3 #include <stack>
 4 #include <cstring>
 5 using namespace std;
 6 const int maxn = 10010;
 7 int dfn[maxn],low[maxn],head[maxn],belong[maxn],scc,tot,idx;
 8 int outde[maxn];
 9 bool instack[maxn];
10 stack<int>stk;
11 struct arc{
12     int to,next;
13     arc(int x = 0,int y = -1){
14         to = x;
15         next = y;
16     }
17 }e[200000];
18 void add(int u,int v){
19     e[tot] = arc(v,head[u]);
20     head[u] = tot++;
21 }
22 void tarjan(int u){
23     dfn[u] = low[u] = idx++;
24     stk.push(u);
25     instack[u] = true;
26     for(int i = head[u]; ~i; i = e[i].next){
27         if(dfn[e[i].to] == -1){
28             tarjan(e[i].to);
29             low[u] = min(low[u],low[e[i].to]);
30         }else if(instack[e[i].to]) low[u] = min(low[u],dfn[e[i].to]);
31     }
32     if(dfn[u] == low[u]){
33         scc++;
34         int v;
35         do{
36             v = stk.top();
37             stk.pop();
38             instack[v] = false;
39             belong[v] = scc;
40         }while(v != u);
41     }
42 }
43 int main(){
44     int n,m,u,v;
45     while(~scanf("%d %d",&n,&m)){
46         memset(head,-1,sizeof(head));
47         memset(belong,0,sizeof(belong));
48         memset(dfn,-1,sizeof(dfn));
49         memset(low,-1,sizeof(low));
50         memset(outde,0,sizeof(outde));
51         while(!stk.empty()) stk.pop();
52         tot = scc = idx = 0;
53         for(int i = 0; i < m; ++i){
54             scanf("%d %d",&u,&v);
55             add(u,v);
56         }
57         for(int i = 1; i <= n; ++i)
58             if(dfn[i] == -1) tarjan(i);
59         for(int i = 1; i <= n; ++i)
60         for(int j = head[i]; ~j; j = e[j].next)
61             if(belong[i] != belong[e[j].to]) outde[belong[i]]++;
62         int ans = 0,p = 0;
63         for(int i = 1; i <= scc; ++i)
64             if(!outde[i]) p = i,ans++;
65         if(ans == 1){
66             ans = 0;
67             for(int i = 1; i <= n; ++i) if(belong[i] == p) ans++;
68             printf("%d\n",ans);
69         }else puts("0");
70     }
71     return 0;
72 }