【模板】有向图强连同分量

 1 # include<cstdio>
 2 # include<cstring>
 3 # include<stack>
 4 # include<algorithm>
 5 using namespace std;
 6 const int N=100000;
 7 const int M=500000;
 8 int n,m,dfs_clock,ecnt,scc_cnt;
 9 int fist[N],next[M],v[M],pre[N],low[N],scc_no[N];
10 stack<int>S;
11 void built(int a,int b){
12     v[++ecnt]=b;
13     next[ecnt]=fist[a];
14     fist[a]=ecnt;
15 }
16 void init(){
17     int a,b;
18     memset(fist,-1,sizeof(fist));
19     scanf("%d%d",&n,&m);
20     for(int i=1;i<=m;i++){
21         scanf("%d%d",&a,&b);
22         built(a,b);
23     }
24 }
25 int dfs(int u){
26     int lowu=pre[u]=++dfs_clock;
27     int lowv;
28     S.push(u);
29     for(int e=fist[u];e!=-1;e=next[e])
30     if(!pre[v[e]])
31     lowu=min(lowu,lowv=dfs(v[e]));
32     else if(!scc_no[v[e]])
33     lowu=min(lowu,pre[v[e]]);
34     low[u]=lowu;
35     if(pre[u]==low[u]){
36         scc_cnt++;
37         for(;;){
38             int x=S.top();S.pop();
39             scc_no[x]=scc_cnt;
40             if(x==u)break;
41         }
42     }
43     return low[u];
44 }
45 void find_scc(){
46     memset(pre,0,sizeof(pre));
47     memset(scc_no,0,sizeof(scc_no));
48     for(int i=1;i<=n;i++)
49     if(!pre[i])dfs(i);
50 }
51 void print(){
52     printf("%d",scc_cnt);
53 }
54 int main(){
55     init();
56     find_scc();
57     print();
58     return 0;
59 }