1 #include<bits/stdc++.h>
2 using namespace std;
3 const int maxn= 100005 ;
4 struct edge{
5 int from ;
6 int to;
7 int next;
8 }EDGE[maxn];
9 vector< int > vc[maxn];
10 int head[maxn],dfn[maxn],vis[maxn],low[maxn],col[maxn], in [maxn],en[maxn],stk[maxn];//each The meaning of the variable can refer to the previous blog
11 int edge_cnt= 1 , tot1= 1,tot2=0,scc_cnt=0,tot0=0;
12 void add(int x,int y)
13 {
14 EDGE[edge_cnt].from=x;
15 EDGE[edge_cnt].to=y;
16 EDGE[edge_cnt].next=head[x];
17 head[x]=edge_cnt++;
18 }
19 void Tarjan(int u)
20 {
21 low[u]=dfn[u]=tot1++;
22 vis[u]=1;
23 stk[++tot2]=u;
24 for(int i = head[u]; i != -1 ; i = EDGE[i].next)
25 {
26 if(!dfn[EDGE[i].to]){
27 Tarjan(EDGE[i].to);
28 low[u]=min(low[u],low[EDGE[i].to]);
29 }
30 else if(vis[EDGE[i].to]){
31 low[u]=min(low[u],low[EDGE[i].to]);
32 }
33 }
34 if(low[u]==dfn[u]){
35 int xx;
36 scc_cnt++;
37 do{
38 xx=stk[tot2--];
39 vc[scc_cnt].push_back(xx);
40 col[xx]=scc_cnt;
41 vis[xx]=0;
42 }while(xx!=u);
43 }
44 }
45 int main()
46 {
47 int n,m;
48 scanf("%d%d",&n,&m);
49 memset(head,-1,sizeof(head));
50 memset(in,0,sizeof(in));
51 while(m--)
52 {
53 int a,b;
54 scanf("%d%d",&a,&b);
55 add(a,b);
56 }
57 for(int i = 1 ; i <= n; i++)
58 {
59 if(!dfn[i])Tarjan(i);
60 }
61 for(int i = 1 ; i <= edge_cnt ; i++)
62 {
63 if(col[EDGE[i].from]!=col[EDGE[i].to])
64 {
65 in[col[EDGE[i].to]]++;//缩点
66 }
67 }
68 for(int i = 1 ; i <= scc_cnt ; i++)
69 {
70 if(in[i])
71 continue;
72 int mmin=vc[i][0];
73 for(int j = 1 ; j < vc[i].size() ; j++)
74 {
75 if(vc[i][j]<mmin)
76 mmin=vc[i][j];
77 }
78 en[tot0++]=mmin;
79 }
80
81 printf("%d\n",tot0);
82 sort(en,en+tot0);
83 for(int i = 0 ; i < tot0 ; i++)
84 {
85 printf("%d",en[i]);
86 char c=(i==tot0-1)?'\n':' ';
87 printf("%c",c);
88 }
89 return 0;
90 }
91 /*4 5
92 1 3
93 2 4
94 4 2
95 1 4
96 2 1*/