poj 2186 Popular Cows

http://poj.org/problem?id=2186

  1 #include <cstdio>

  2 #include <cstring>

  3 #include <algorithm>

  4 #include <vector>

  5 #include <stack>

  6 #define maxn 100001

  7 using namespace std;

  8 

  9 int dfn[maxn],low[maxn],belong[maxn],bcnt,bcc_clock,a[maxn],b[maxn],num[maxn],n,m,head[maxn],e;

 10 stack<int>s;

 11 bool vis[maxn],flag[maxn];

 12 struct node

 13 {

 14     int v,next;

 15 }p[maxn];

 16 

 17 

 18 void add(int u,int v)

 19 {

 20     p[e].v=v;

 21     p[e].next=head[u];

 22     head[u]=e++;

 23 }

 24 void tarjan(int u)

 25 {

 26     dfn[u]=low[u]=++bcc_clock;

 27     vis[u]=true;

 28     s.push(u);

 29     for(int i=head[u]; i!=-1; i=p[i].next)

 30     {

 31         int v=p[i].v;

 32         if(!dfn[v])

 33         {

 34             tarjan(v);

 35             low[u]=min(low[u],low[v]);

 36         }

 37         else if(vis[v]&&dfn[v]<low[u])

 38         low[u]=dfn[v];

 39     }

 40     if(dfn[u]==low[u])

 41     {

 42         bcnt++;

 43         int j;

 44         do

 45         {

 46             j=s.top();

 47             s.pop();

 48             vis[j]=false;

 49             belong[j]=bcnt;

 50         }while(j!=u);

 51     }

 52 }

 53 

 54 void deal()

 55 {

 56     while(!s.empty())

 57     {

 58         s.pop();

 59     }

 60     memset(dfn,0,sizeof(dfn));

 61     bcnt=0;bcc_clock=0;

 62     for(int i=1; i<=n; i++)

 63     {

 64         if(!dfn[i])

 65         {

 66             tarjan(i);

 67         }

 68     }

 69 }

 70 

 71 void inti()

 72 {

 73     e=0;

 74     memset(head,-1,sizeof(head));

 75     memset(belong,0,sizeof(belong));

 76     memset(low,0,sizeof(low));

 77     memset(a,0,sizeof(a));

 78     memset(b,0,sizeof(b));

 79     memset(num,0,sizeof(num));

 80     memset(flag,false,sizeof(flag));

 81     memset(vis,false,sizeof(vis));

 82 }

 83 

 84 int main()

 85 {

 86     while(scanf("%d%d",&n,&m)!=EOF)

 87     {

 88         inti();

 89         for(int i=1; i<=m; i++)

 90         {

 91             scanf("%d%d",&a[i],&b[i]);

 92             add(a[i],b[i]);

 93         }

 94         deal();

 95         for(int i=1; i<=m; i++)

 96         {

 97             if(belong[a[i]]!=belong[b[i]])

 98                 flag[belong[a[i]]]=true;

 99         }

100         for(int i=1; i<=n; i++)

101         {

102             num[belong[i]]++;

103         }

104         int t=0,ans;

105         for(int i=1; i<=bcnt; i++)

106         {

107             if(!flag[i])

108             {

109                 t++;

110                 ans=num[i];

111             }

112         }

113         if(t==1)

114             printf("%d\n",ans);

115         else

116             printf("0\n");

117     }

118     return 0;

119 }

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