ZJNU 1269 - Lighthouse - High

The subject can be an input to the FIG.

Signal may have passed to the transfer of the graph, it can be said that there can be found the father figure to all

But also consider this type of situation can occur ring

Run again so strongly connected component template, then according to block the view of the degree to find block 0, the source signal is issued as the blocks, you can make full picture signal can be received

Therefore, the answer is a block of the number 0

(Because the program ran just stuck a time limit, so use a buffer read optimization, the final program takes 78ms)

 1 /*
 2 Written By StelaYuri
 3 */
 4 #pragma GCC optimize(3)
 5 #include<bits/stdc++.h>
 6 using namespace std;
 7 const int N=100005;
 8 vector<int> G[N];
 9 stack<int> S;
10 int pre[N],lowlink[N],sccno[N],dfs_clock,scc_cnt,ind[N];
11 const int bsz=1<<16;
12 char bf[bsz],*head,*tail;
13 inline char gc(){
14     if(head==tail){
15         int l=fread(bf,1,bsz,stdin);
16         tail=(head=bf)+l;
17     }
18     return *head++;
19 }
20 inline int read(){
21     int x=0;char c=gc();
22     while(!isdigit(c))c=gc();
23     for(;isdigit(c);c=gc())x=x*10+c-'0';
24     return x;
25 }
26 inline void write(int x){
27     if(x>=10)write(x/10);
28     putchar(x%10+'0');
29 }
30 void dfs(int in){
31     pre[in]=lowlink[in]=++dfs_clock;
32     S.push(in);
33     int num=G[in].size();
34     for(int i=0;i<num;i++){
35         int v=G[in][i];
36         if(!pre[v]){
37             dfs(v);
38             lowlink[in]=min(lowlink[in],lowlink[v]);
39         }
40         else if(!sccno[v])
41             lowlink[in]=min(lowlink[in],pre[v]);
42     }
43     if(lowlink[in]==pre[in]){
44         scc_cnt++;
45         while(1){
46             int x=S.top();
47             S.pop();
48             sccno[x]=scc_cnt;
49             if(x==in)
50                 break;
51         }
52     }
53 }
54 int main(){
55     int N=read(),M=read(),i,j,a,b;
56     for(i=0;i<M;i++){
57         a=read();
58         b=read();
59         G[a].push_back(b);
60     }
61     dfs_clock=scc_cnt=0;
62     memset(sccno,0,sizeof sccno);
63     memset(pre,0,sizeof pre);
64     for(i=1;i<=N;i++)
65         if(!pre[i])
66             dfs(i);
67     memset(ind,0,sizeof ind);
68     for(i=1;i<=N;i++){
69         a=G[i].size();
70         for(j=0;j<a;j++)
71             if(sccno[i]!=sccno[G[i][j]])
72                 ind[sccno[G[i][j]]]++;
73     }
74     for(a=0,i=1;i<=scc_cnt;i++)
75         if(!ind[i])
76             a++;
77     write(a);
78     
79     return 0;
80 }