1 #include<iostream> 2 #include<cstring> 3 #include<vector> 4 using namespace std; 5 const int maxn=1024; 6 struct Edge{ 7     int go,next; 8 }; 9 int vis[maxn],count=0,book[maxn];10 vector<Edge> G,G2;11 vector<int> S;12 int end[maxn],end2[maxn];13 void add(int from,int to){Edge e;e.go=to;e.next=end[from];G.push_back(e);end[from]=G.size()-1;}14 void add2(int from,int to){Edge e;e.go=to;e.next=end2[from];G2.push_back(e);end2[from]=G2.size()-1;}15 void dfs(int u)16 {17     vis[u]=1;18     for(int i=end[u];i;i=G[i].next){19         int go=G[i].go;20         if(!vis[go]) dfs(go);21     }22     S.push_back(u);23 }24 void dfs2(int u)25 {26     book[u]=count;27     for(int i=end2[u];i;i=G2[i].next){28         int go=G2[i].go;29         if(!book[go]) dfs2(go);30     }31 }32 void init()33 {34     memset(vis,0,sizeof(vis));35     memset(end,0,sizeof(end));36     memset(end2,0,sizeof(end2));37     memset(book,0,sizeof(book));38 }39 int main()40 {41     int n,m,a,b;42     scanf("%d %d",&n,&m);43     init();44     for(int i=1;i<=m;i++)45     {46         scanf("%d %d",&a,&b);47         add(a,b);48         add2(b,a);49     }50     for(int i=1;i<=n;i++) if(!vis[i]) dfs(i);51     for(int i=n-1;i>=0;i--) if(!book[S[i]]){52         count++;53         dfs2(S[i]);54     }55     cout<<count;56     return 0;57 }