Tarjan+模板

 #include<stdio.h>
 #include<string.h>
 #include<stdlib.h>
 #include<math.h>
 #include<algorithm>
 #include<iostream>
 #include<set>
 #include<stack>
 #include<queue>
 #include<map>
 #include<vector>
 using namespace std;

 const int maxn = ;
 const int maxm = ;
 const int inf = 0x3f3f3f3f;
 const double pi = acos(-1.0);
 const double eps = 1e-;

 struct Edge{
     int u,v,next;
 }edge[ maxn*maxn ];
 int cnt,head[ maxn ];
 int vis[ maxn ],belong[ maxn ];
 stack<int>s;
 int dfn[ maxn ],low[ maxn ];
 int Cnt ,id ;

 void init(){
     while( !s.empty() )
         s.pop();
     Cnt = ;
     id = ;
     memset( dfn,-,sizeof( dfn ) );
     memset( low,-,sizeof( low ) );
     cnt = ;
     memset( head,-,sizeof( head ) );
 }

 void addedge( int a,int b ){
     edge[ cnt ].u = a;
     edge[ cnt ].v = b;
     edge[ cnt ].next = head[ a ];
     head[ a ] = cnt++;
 }

 void tarjan( int cur ){
     dfn[ cur ] = low[ cur ] = id++;
     vis[ cur ] = ;
     s.push( cur );
     for( int i=head[ cur ];i!=-;i=edge[i].next ){
         int nxt = edge[i].v;
         if( dfn[ nxt ]==- ){
             tarjan( nxt );
             low[ cur ] = min( low[ nxt ],low[ cur ] );
         }
         else if( vis[ nxt ]== ){
             low[ cur ] = min( dfn[ nxt ],low[ cur ] );
         }
     }
     if( dfn[ cur ]==low[ cur ] ){
         Cnt ++;
         while(  ){
             int tmp = s.top();
             s.pop();
             vis[ tmp ] = ;
             belong[ tmp ] = Cnt;
             if( tmp==cur ) break;
         }
     }
 }

 int main(){
     int n,m;
     while( scanf("%d%d",&n,&m)== ){
         int a,b;
         init();
         while( m-- ){
             scanf("%d%d",&a,&b);
             addedge( a,b );
         }
         for( int i=;i<=n;i++ ){
             if( dfn[i]==- ){
                 tarjan( i );
             }
         }
         printf("缩点 = %d\n",Cnt);
     }
     return ;
 }