HDU1269 (tarjan)

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 1 #include<stdio.h>

 2 #include<stack>

 3 #include<string.h>

 4 #include<stdlib.h>

 5 #include<algorithm>

 6 using namespace std;

 7 const int maxn = 10005;

 8 const int maxm = 100005;

 9 struct node{

10     int u,next;

11 }edge[ maxm ];

12 int head[ maxn ],cnt;

13 int vis[ maxn ],dfn[ maxn ],low[ maxn ],cnt2,id;

14 int n,m;

15 

16 void init(){

17     cnt=0;

18     id=0;

19     cnt2=0;

20     memset( head,-1,sizeof( head ));

21     memset( vis,0,sizeof( vis ));

22     memset( dfn,-1,sizeof( dfn ));

23     memset( low,0,sizeof( low ));

24 }

25 void addedge( int a,int b ){

26     edge[ cnt ].u=b;

27     edge[ cnt ].next=head[ a ];

28     head[ a ]=cnt++;

29 }

30 stack<int>s;

31 

32 void tarjan( int now ){

33     dfn[ now ]=low[ now ]=id++;

34     vis[ now ]=1;

35     s.push( now );

36     for( int i=head[ now ];i!=-1;i=edge[ i ].next ){

37         int next=edge[ i ].u;

38         if( dfn[ next ]==-1 ){

39             tarjan( next );

40             low[ now ]=min( low[ now ],low[ next ] );

41         }

42         else if( vis[ next ]==1 ){

43             low[ now ]=min( low[ now ],dfn[ next ] );

44         }

45     }

46     if( dfn[ now ]==low[ now ] ){

47         cnt2++;

48         while( 1 ){

49             int tmp=s.top();

50             s.pop();

51             vis[ tmp ]=0;

52             if( tmp==now ) break;

53         }

54     }

55 }

56 

57 int main(){

58     while( scanf("%d%d",&n,&m)==2 &&(n+m) ){

59         init();

60         int a,b;

61         while( m-- ){

62             scanf("%d%d",&a,&b);

63             addedge( a,b );

64         }

65         for( int i=1;i<=n;i++ )

66             if( dfn[ i ]==-1 )

67                 tarjan( i );

68         if( cnt2==1 ) printf("Yes\n");

69         else printf("No\n");

70     }

71     return 0;

72 }