1 #include<iostream>
2 #include<cstring>
3 #include<algorithm>
4 #include<queue>
5 #include<stack>
6 #include<vector>
7 using namespace std;
8 const int maxn=100005;//边数
9 const int maxn1=1005;//顶点数
10 struct edge{
11 int from;
12 int to;
13 int next;
14 }EDGE[maxn];
15 vector<int>vc[maxn1];
16 int head[maxn1],dfn[maxn1],vis[maxn1],low[maxn1],col[maxn1],out[maxn1],in[maxn1],en[maxn1],stk[maxn1];//各个变量的意义可参照上篇博客
17 int edge_cnt=0,tot1=0,tot2=0,scc_cnt=0,tot0=0;
18 void add(int x,int y)
19 {
20 EDGE[edge_cnt].from=x;
21 EDGE[edge_cnt].to=y;
22 EDGE[edge_cnt].next=head[x];
23 head[x]=edge_cnt++;
24 }
25 void Tarjan(int u)
26 {
27 low[u]=dfn[u]=++tot1;//注意tot1的初值必须是1【因为dfn必须为正数】，所以这里使用++tot1而不用tot1++;
28 vis[u]=1;
29 stk[++tot2]=u;
30 for(int i = head[u]; i != -1 ; i = EDGE[i].next)
31 {
32 if(!dfn[EDGE[i].to]){
33 Tarjan(EDGE[i].to);
34 low[u]=min(low[u],low[EDGE[i].to]);
35 }
36 else if(vis[EDGE[i].to]){
37 low[u]=min(low[u],low[EDGE[i].to]);
38 }
39 }
40 if(low[u]==dfn[u]){
41 int xx;
42 scc_cnt++;//注意scc_cnt也是从1开始的，因为要染色，区别于为染色的0
43 do{
44 xx=stk[tot2--];
45 vc[scc_cnt].push_back(xx);
46 col[xx]=scc_cnt;
47 vis[xx]=0;
48 }while(xx!=u);
49 }
50 }
51 void INIT()
52 {
53 for(int i = 0 ; i < maxn1 ; i++)
54 vc[i].clear();
55 edge_cnt=0,tot1=0,tot2=0,scc_cnt=0,tot0=0;
56 memset(head,-1,sizeof(head));
57 memset(stk,0,sizeof(stk));
58 memset(in,0,sizeof(in));
59 memset(out,0,sizeof(out));
60 memset(dfn,0,sizeof(dfn));
61 memset(low,0,sizeof(low));
62 memset(col,0,sizeof(col));
63 }
64 void suodian()//缩点
65 {
66 for(int i = 0 ; i < edge_cnt ; i++)
67 {
68 if(col[EDGE[i].from]!=col[EDGE[i].to])
69 {
70 in[col[EDGE[i].to]]++;//缩点
71 out[col[EDGE[i].from]]++;
72 }
73 }
74 }
75 int main()
76 {
77 int n,m;
78 scanf("%d%d",&n,&m);
79 INIT();
80 while(m--)
81 {
82 int a,b;
83 scanf("%d%d",&a,&b);
84 add(a,b);
85 }
86 for(int i = 1 ; i <= n; i++)
87 {
88 if(!dfn[i])Tarjan(i);
89 }
90 suodian();
91 return 0;
92 }
93 /*4 5
94 1 3
95 2 4
96 4 2
97 1 4
98 2 1*/
