[BZOJ] 1051: [HAOI2006]受欢迎的牛

 1 #include<cstdio>
 2 #include<iostream>
 3 #define maxn 1000000
 4 using namespace std;
 5 
 6 int n,m,de[maxn],ret,ans;
 7 int dfn[maxn],low[maxn],TIM,stack[maxn],pos,COL,buck[maxn],color[maxn];
 8 bool vis[maxn];
 9 
10 struct edge{ int from,u,v; }e[maxn],E[maxn];
11 int tot,first[maxn],toT,firsT[maxn];
12 void insert(int u,int v){ tot++; e[tot].from = first[u]; e[tot].u = u; e[tot].v = v; first[u] = tot; }
13 void inserT(int u,int v){ toT++; E[toT].from = firsT[u]; E[toT].u = u; E[toT].v = v; firsT[u] = toT; }
14 void tarjan(int now){
15     dfn[now] = low[now] = ++TIM;
16     stack[pos++] = now; vis[now] = true;
17     for(int i = first[now];i;i = e[i].from){
18         int v = e[i].v;
19         if(!dfn[v]){
20             tarjan(v); low[now] = min(low[now],low[v]);
21         }else if(vis[v]) low[now] = min(low[now],dfn[v]);
22     }if(dfn[now] == low[now]){
23         COL++;
24         while(stack[pos-1] != now && pos){
25             buck[COL]++;
26             color[stack[pos-1]] = COL;
27             vis[stack[--pos]] = false;
28         }color[now] = COL; vis[now] = false; pos--; buck[COL]++;
29     }
30 }
31 
32 int main(){
33     scanf("%d%d",&n,&m);
34     
35     for(int i = 1;i <= m;i++){
36         int x,y; scanf("%d%d",&x,&y);
37         insert(x,y);
38     }for(int i = 1;i <= n;i++) if(!dfn[i]) tarjan(i);
39     
40     for(int i = 1;i <= tot;i++){
41         int u = e[i].u,v = e[i].v;
42         if(color[u] != color[v]){
43             inserT(color[u],color[v]);
44             de[color[u]]++;
45         }
46     }
47     
48     for(int i = 1;i <= COL;i++){
49         if(!de[i]) ret++,ans = buck[i];
50     }
51     
52     if(ret > 1) cout << 0;
53     else cout << ans;
54     
55     return 0;
56 }