poj2186 Popular Cows 题解——S.B.S.

  1 #include<iostream>
  2 #include<cstdio>
  3 #include<cstring>
  4 #include<cmath>
  5 #include<algorithm>
  6 #include<queue>
  7 #include<cstdlib>
  8 #include<iomanip>
  9 #include<cassert>
 10 #include<climits>
 11 #define MAXN 10001
 12 #define MAXM 50001
 13 #define inf 0x7fffffff
 14 #define F(i,j,k) for(int i=j;i<=k;i++)
 15 #define M(a,b) memset(a,b,sizeof(a))
 16 using namespace std;
 17 inline int read(){
 18     int x=0,f=1;char ch=getchar();
 19     while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
 20     while(ch>=‘0‘&&ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}
 21     return x*f;
 22 }
 23 struct node  
 24 {  
 25     int to,next;  
 26 }edge[MAXM];  
 27 int n,m,head[MAXN],dfn[MAXN],low[MAXN],stack1[MAXN],num[MAXN],du[MAXN],vis[MAXN],cnt,time=1,top,cut;  
 28 int tim,tp,dcnt,sum;
 29 void addedge(int u,int v)  
 30 {  
 31     edge[cnt].to=v;  
 32     edge[cnt].next=head[u];  
 33     head[u]=cnt;  
 34     cnt++;  
 35 }
 36 void dfs(int u,int fa)  
 37 {  
 38     dfn[u]=time;  
 39     low[u]=time;  
 40     time++;  
 41     vis[u]=1;  
 42     stack1[top]=u;  
 43     top++;  
 44     for(int i=head[u]; i!=-1; i=edge[i].next)  
 45     {  
 46         int v=edge[i].to;  
 47         if(!vis[v])  
 48         {  
 49             dfs(v,u);  
 50             low[u]=min(low[u],low[v]);  
 51         }  
 52         else if(vis[v])  
 53         {  
 54             low[u]=min(low[u],dfn[v]);  
 55         }  
 56     }  
 57     if(low[u]==dfn[u])  
 58     {  
 59         cut++;  
 60         while(top>0&&stack1[top]!=u)  
 61         {  
 62             top--;  
 63             vis[stack1[top]]=2;  
 64             num[stack1[top]]=cut;  
 65         }  
 66     }  
 67 }  
 68 int main()
 69 {
 70     std::ios::sync_with_stdio(false);//cout<<setiosflags(ios::fixed)<<setprecision(1)<<y;
 71 //  freopen("data.in","r",stdin);
 72 //  freopen("data.out","w",stdout);
 73 //    int i,u,v;
 74     cin>>n>>m;
 75     M(head,-1);
 76     F(i,0,m-1)
 77     {
 78         int a,b;
 79         cin>>a>>b;
 80         addedge(a,b);
 81     }
 82     F(i,1,n)
 83     {
 84         if(!vis[i]) dfs(i,0);
 85     }
 86     F(i,1,n)for(int j=head[i];j!=-1;j=edge[j].next)
 87     {
 88         if(num[i]!=num[edge[j].to])
 89         {
 90             du[num[i]]++;
 91         }
 92     }
 93     int x;sum=0;
 94     F(i,1,cut)
 95     {
 96         if(!du[i])
 97         {
 98             sum++;
 99             x=i;
100         }
101     }
102     if(sum==1)
103     {
104         sum=0;
105         F(i,1,n)
106         {
107             if(num[i]==x)
108             {
109                 sum++;
110             }
111         }
112         cout<<sum<<endl;
113     }
114     else cout<<"0"<<endl;
115     return 0;
116 }