题目描述 Description

输入描述 Input Description

输出描述 Output Description

样例输入 Sample Input

样例输出 Sample Output

#include <cstdio>
#include <cmath>
#include <cstring>
#include <ctime>
#include <iostream>
#include <algorithm>
#include <set>
#include <vector>
#include <queue>
#include <typeinfo>
#include <map>
#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
#define inf 10000000
inline ll read()
{ll x=0,f=1;char ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}return x*f;
}
//***************************************************************struct ss
{int u,to,next;
} e[50001];
int out[40001];
int head[50001],vis[50001],dfn[50001],low[50001],belong[50001],hav[50001];
int q[50001],inq[50001];
int t=1,bcnt,cnt,n,m,top;
void add(int u,int v)
{e[t].to=v;e[t].u=u;e[t].next=head[u];head[u]=t++;
}
void dfs(int u)
{vis[u]=inq[u]=1;low[u]=dfn[u]=++cnt;q[++top]=u;for(int i=head[u]; i; i=e[i].next){if(!vis[e[i].to]){dfs(e[i].to);low[u]=min(low[u],low[e[i].to]);}else if(inq[e[i].to])low[u]=min(low[u],dfn[e[i].to]);}int v=-1;if(low[u]==dfn[u]){bcnt++;while(v!=u){v=q[top--];inq[v]=0;belong[v]=bcnt;hav[bcnt]++;}}
}
void rebuild()
{for(int i=1; i<=m; i++){if(belong[e[i].u]!=belong[e[i].to]){out[belong[e[i].u]]++;}}
}
void tarjan()
{for(int i=1; i<=n; i++){if(!vis[i]){dfs(i);}}rebuild();
}
void work()
{int res=0;int ans;for(int i=1; i<=bcnt; i++){if(out[i]==0){ans=i;res++;}}int bb=0;for(int i=1;i<=bcnt;i++){if(hav[i]>=2)bb++;}cout<<bb<<endl;if(res==1){if(hav[ans]!=1){int kk=0;int hh[50001];for(int i=1; i<=n; i++){if(belong[i]==ans){hh[++kk]=i;}}for(int i=1; i<kk; i++){printf("%d ",hh[i]);}cout<<hh[kk]<<endl;}else cout<<-1<<endl;}else cout<<-1<<endl;
}
int main()
{int u,v;scanf("%d%d",&n,&m);for(int i=1; i<=m; i++){scanf("%d%d",&u,&v);add(u,v);}tarjan();work();return 0;
}