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| #include<cstdio>
#include<vector>
using namespace std;
int top=0;
int dfn[99999];
int low[99999];
int colour[99999];
int dfn_num;
int ans;
int colour_num;
int vis[99999];
int strak[99999];
int f[1999][1999];
int r[999999];
int flag[99999];
vector < int > a[99999];
vector < int > b[99999];
void dfs(int x)
{
dfn[x]=++dfn_num;
low[x]=dfn_num;
vis[x]=1;
strak[++top]=x;
for(int i=0;i<a[x].size();i++)
{
int temp=a[x][i];
if(!dfn[temp])
{
dfs(temp);
low[x]=min(low[x],low[temp]);
}
else if(vis[temp])
{
low[x]=min(low[x],low[temp]);
}
}
if(dfn[x]==low[x])
{
vis[x]=0;
int t=0;
colour[x]=++colour_num;
while(strak[top]!=x)
{
vis[strak[top]]=0;
colour[strak[top--]]=colour_num;
t++;
}
top--;
if(t>=1) ans++,flag[colour_num]=1;
}
}
int n,m;
int main()
{
scanf("%d%d",&n,&m);
for(int i=1;i<=m;i++)
{
int x,y;
scanf("%d%d",&x,&y);
a[x].push_back(y);
}
for(int i=1;i<=n;i++)
if(!dfn[i])
dfs(i);
printf("%d\n",ans);
//for(int i=1;i<=n;i++)
// printf("%d ",colour[i]);
ans=0;int z;
for(int j=1;j<=n;j++)
{
for(int i=0;i<a[j].size();i++)
if(f[colour[j]][colour[a[j][i]]]==0&&colour[j]!=colour[a[j][i]])
{
b[colour[j]].push_back(colour[a[j][i]]);//缩点
f[colour[j]][colour[a[j][i]]]=1;
r[colour[j]]++;
}
}
for(int i=1;i<=colour_num;i++)
{
if(r[i]==0&&flag[i]==1) //找出度为零的点
{
z=i;
ans++;
}
}
if(ans==1)
{
for(int j=1;j<=n;j++)
if(colour[j]==z)
printf("%d ",j);//输出
}
//for(int i=1;i<=colour)
else printf("-1");
return 0;
}
|
