poj 2486 有向图强连通分量

  1 #include<cstdio>
  2 #include<iostream>
  3 #include<cstring>
  4 const int MAXN=20010;//点数
  5 const int MAXM=50010;//边数
  6 struct Edge
  7 {
  8     int to,next;
  9 }edge[MAXM];
 10 int head[MAXN],tot;
 11 int Low[MAXN],DFN[MAXN],Stack[MAXN],Belong[MAXN];//Belong数组的值是1~scc
 12 int Index,top;
 13 int scc;//强连通分量的个数
 14 bool Instack[MAXN];
 15 int num[MAXN];//各个强连通分量包含点的个数，数组编号1~scc
 16 //num数组不一定需要，结合实际情况
 17 int out[MAXN],tmp,Num,ans;
 18 void addedge(int u,int v)
 19 {
 20     edge[tot].to=v;edge[tot].next=head[u];head[u]=tot++;
 21 }
 22 void Tarjan(int u)
 23 {
 24     int v;
 25     Low[u]=DFN[u]=++Index;
 26     Stack[top++]=u;
 27     Instack[u]=true;
 28     for(int i=head[u];i != -1;i=edge[i].next)
 29     {
 30         v=edge[i].to;
 31         if(!DFN[v])
 32         {
 33             Tarjan(v);
 34             if(Low[u] > Low[v])Low[u]=Low[v];
 35         }
 36         else if(Instack[v] && Low[u] > DFN[v])
 37         Low[u]=DFN[v];
 38     }
 39     if(Low[u]==DFN[u])
 40     {
 41         scc++;
 42         do
 43         {
 44             v=Stack[--top];
 45             Instack[v]=false;
 46             Belong[v]=scc;
 47             num[scc]++;
 48         }
 49         while(v != u);
 50     }
 51 }
 52 void solve(int N)
 53 {
 54     memset(out,0,sizeof(out));
 55     memset(Belong,0,sizeof(Belong));
 56     memset(DFN,0,sizeof(DFN));
 57     memset(Instack,false,sizeof(Instack));
 58     memset(num,0,sizeof(num));
 59     Index=scc=top=0;
 60     for(int i=1;i <= N;i++)
 61         if(!DFN[i])
 62             Tarjan(i);
 63 }
 64 void init()
 65 {
 66     tot=0;
 67     memset(head,-1,sizeof(head));
 68 }
 69 int main()
 70 {
 71     int n,m;
 72     int i,j,v;
 73     //freopen("1.in","r",stdin);
 74     while(scanf("%d%d",&n,&m)!=EOF)
 75     {
 76         init();
 77         int q,p;
 78         for(i=1;i<=m;i++)
 79         {
 80             scanf("%d%d",&p,&q);
 81             addedge(p,q);
 82         }
 83         solve(n);
 84         for(i=1;i<=n;i++)
 85         {
 86             for(v=head[i];v!=-1;v=edge[v].next)
 87             {
 88                 if(Belong[i]!=Belong[edge[v].to])
 89                 {
 90                     out[Belong[i]]++;
 91                 }
 92             }
 93         }
 94         ans=0,Num=0;
 95         for(i=1;i<=scc;i++)
 96         {
 97             if(!out[i])
 98             {
 99                 Num++;
100                 tmp = i;
101             }
102         }
103         if(Num==1)
104         {
105             for(i=1;i<=n;i++)
106             {
107                 if(Belong[i]==tmp)
108                     ans++;
109             }
110             printf("%d\n",ans);
111         }
112         else
113         {
114             printf("0\n");
115         }
116     }
117     return 0;
118 }