[洛谷2002]消息扩散

 1 #include<stack>
 2 #include<cstdio>
 3 #include<cctype>
 4 #include<vector>
 5 inline int getint() {
 6     char ch;
 7     while(!isdigit(ch=getchar()));
 8     int x=ch^‘0‘;
 9     while(isdigit(ch=getchar())) x=(((x<<2)+x)<<1)+(ch^‘0‘);
10     return x;
11 }
12 const int V=100001;
13 std::vector<int> e[V];
14 inline void add_edge(const int u,const int v) {
15     e[u].push_back(v);
16 }
17 int dfn[V]={0},low[V]={0},scc[V]={0},cnt=0,id=0;
18 bool ins[V]={0};
19 std::stack<int> s;
20 void Tarjan(const int x) {
21     dfn[x]=low[x]=++cnt;
22     s.push(x);
23     ins[x]=true;
24     for(unsigned i=0;i<e[x].size();i++) {
25         int &y=e[x][i];
26         if(!dfn[y]) {
27             Tarjan(y);
28             low[x]=std::min(low[x],low[y]);
29         }
30         else if(ins[y]) {
31             low[x]=std::min(low[x],dfn[y]);
32         }
33     }
34     if(dfn[x]==low[x]) {
35         int y;
36         ++id;
37         do {
38             y=s.top();
39             s.pop();
40             ins[y]=false;
41             scc[y]=id;
42         } while(y!=x);
43     }
44 }
45 int in[V]={0};
46 int main() {
47     int n=getint();
48     for(int m=getint();m;m--) {
49         int u=getint(),v=getint();
50         add_edge(u,v);
51     }
52     for(int i=1;i<=n;i++) {
53         if(!dfn[i]) Tarjan(i);
54     }
55     for(int x=1;x<=n;x++) {
56         for(unsigned i=0;i<e[x].size();i++) {
57             int &y=e[x][i];
58             if(scc[x]!=scc[y]) {
59                 in[scc[y]]++;
60             }
61         }
62     }
63     int ans=0;
64     for(int i=1;i<=id;i++) {
65         if(!in[i]) ans++;
66     }
67     printf("%d\n",ans);
68     return 0;
69 }