Popular Cows

 1 #include<iostream>
 2 #include<algorithm>
 3 #include<vector>
 4 #include<stdio.h>
 5 #include<stdlib.h>
 6 #include<string.h>
 7 #define MaxNum 10005
 8 using namespace std;
 9  
10 int n,m;
11 vector<int > edge[MaxNum];
12 int dfn[MaxNum],low[MaxNum];
13 int time=0;
14 int color=0;
15 int allColor[MaxNum]={0};
16 int allSum[MaxNum]={0};
17 int stack[MaxNum],top=0;
18 bool vis[MaxNum];
19 
20 void targan(int d){
21     vis[d]=true;
22     dfn[d]=++time;
23     low[d]=time;
24     stack[++top]=d;
25     for(int i=0;i<edge[d].size();++i)
26     {
27         if(!vis[edge[d][i]])
28         {
29             targan(edge[d][i]);
30             low[d]=min(low[d],low[edge[d][i]]);
31         }
32         else low[d]=min(low[d],dfn[edge[d][i]]);
33     }
34     
35     if(dfn[d]==low[d])
36     {
37         ++color;
38         while(stack[top]!=d)
39         {
40             allColor[stack[top--]]=color;
41             allSum[color]++;
42         }
43         
44         allColor[stack[top--]]=color;
45         allSum[color]++;
46     }
47 }
48 
49 int out[MaxNum]={0};
50 int main()
51 {
52     cin>>n>>m;
53     for(int i=1;i<=m;++i)
54     {
55         int x,y;
56         cin>>x>>y;
57         edge[x].push_back(y);
58     }
59     
60     memset(vis,false,sizeof(vis));
61     for(int i=1;i<=n;++i)
62     if(!vis[i])
63     targan(i);
64     
65     
66     for(int i=1;i<=n;++i)
67     for(int j=0;j<edge[i].size();++j)
68     if(allColor[i]!=allColor[edge[i][j]])
69     out[allColor[i]]++;
70     
71     int total=0;int ans=0;
72     for(int i=1;i<=color;++i)
73     if(out[i]==0) 
74     {
75         total++;
76         ans=allSum[i];
77     }
78     
79     if(total>1) ans=0;
80     cout<<ans<<endl;
81     return 0;
82 
83 }