codevs 2822 爱在心中

  1 #include<iostream>
  2 using namespace std;
  3 #include<cstdio>
  4 #include<algorithm>
  5 #define N 10000
  6 #include<stack>
  7 stack <int> sta;
  8 bool instack[N]={false};
  9 int n,m;
 10 struct Edge{
 11     int u,v,last;
 12 }edge[N<<1];
 13 int dfn[N],low[N],father[N];
 14 int t=0,sum=0,head[N]={0},topt=0,outdu[N]={0},sumout=0,ans1[N],ans2[N];
 15 void add_edge(int u,int v)
 16 {
 17     edge[++t].v=v;
 18     edge[t].u=u;
 19     edge[t].last=head[u];
 20     head[u]=t;
 21 }
 22 void input()
 23 {
 24     scanf("%d%d",&n,&m);
 25     int u1,v1;
 26     for(int i=1;i<=m;++i)
 27     {
 28         scanf("%d%d",&u1,&v1);
 29         add_edge(u1,v1);
 30     }
 31 }
 32 void tarjan(int k)
 33 {
 34     dfn[k]=low[k]=++topt;
 35     sta.push(k);
 36     instack[k]=true;
 37     for(int l=head[k];l;l=edge[l].last)
 38     {
 39         if(dfn[edge[l].v]==0)
 40         {
 41             tarjan(edge[l].v);
 42             low[k]=min(low[k],low[edge[l].v]);
 43         }
 44         else if(instack[edge[l].v])
 45                 low[k]=min(low[k],dfn[edge[l].v]);
 46     }
 47     if(dfn[k]==low[k])
 48     {
 49         int js=0;
 50         int x;
 51         do
 52         {
 53             x=sta.top();
 54             sta.pop();
 55             father[x]=k;
 56             instack[x]=false;
 57             js++;
 58         }while(x!=k);
 59         if(js>=2)
 60           ans1[++ans1[0]]=k;
 61     }
 62 }
 63 bool work()
 64 {
 65     for(int i=1;i<=m;++i)
 66     {
 67         int x=father[edge[i].u];
 68         int y=father[edge[i].v];
 69         if(x==y) continue;
 70         outdu[x]++;
 71     }
 72     int comp;
 73     for(int i=1;i<=ans1[0];++i)
 74     {
 75         if(outdu[ans1[i]]==0) 
 76         {
 77             sumout++;
 78             comp=ans1[i];
 79         }
 80     }
 81     if(sumout!=1) return false;
 82     for(int i=1;i<=n;++i)
 83     {
 84         if(father[i]==comp)
 85         {
 86             ans2[++ans2[0]]=i;
 87         }
 88     }
 89     return true;
 90 }
 91 int main()
 92 {
 93     input();
 94     for(int i=1;i<=n;++i)
 95     {
 96         if(!dfn[i])
 97           tarjan(i);
 98     }
 99     printf("%d\n",ans1[0]);
100     if(work())
101     {
102         sort(ans2+1,ans2+ans2[0]+1);
103         for(int i=1;i<=ans2[0];++i)
104           printf("%d ",ans2[i]);
105         printf("\n");
106     }else printf("-1\n");
107     return 0;
108 }