HDU1689 BFS+最小奇数环

题意：给定一张无向图，求最小的奇数环。

注意：状态的设定 vis[ i ][ 0 ] and vis[ i ][ 1 ] 很重要！！！

View Code

 1 /*

 2 BFS+最小奇数环

 3 */

 4 #include<stdio.h>

 5 #include<string.h>

 6 #include<stdlib.h>

 7 #include<algorithm>

 8 #include<iostream>

 9 #include<queue>

10 #include<stack>

11 #include<math.h>

12 #include<map>

13 using namespace std;

14 const int maxn = 1005;

15 const int maxm = 40005;

16 const int inf = 0x7fffffff;

17 int head[ maxn ],cnt;

18 struct node{

19     int u,next;

20 }edge[ maxm ];

21 int dis[ maxn ],vis[ maxn ][ 2 ];//vis[ i ][ 0 ],在i点偶数步；dis[ i ]仅代表i的 lev ！！

22 int mat[ maxn ][ maxn ];

23 int ans;

24 void init(){

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

26     cnt=0;

27     memset( mat,-1,sizeof( mat ) );

28     ans=inf;

29 }

30 void addedge( int a,int b ){

31     edge[ cnt ].u=b;

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

33     head[ a ]=cnt++;

34 }

35 void  bfs( int s,int n ){

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

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

38         dis[ i ]=inf;

39     queue<int>q;

40     q.push( s );

41     vis[ s ][ 0 ]=1;

42     dis[ s ]=0;

43     //printf("bfs\n");

44     while( !q.empty() ){

45         int now=q.front();

46         //printf("now:%d\n",now);

47         q.pop();

48         if( vis[ s ][ (dis[s]%2) ]==1 && (dis[ s ]%2)==1 && dis[ s ]>=3 ){

49             ans=min( ans,dis[ s ] );

50         }

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

52             int nxt=edge[ i ].u;

53             //if( dis[ nxt ]>dis[ now ]+1 ){

54             //    dis[ nxt ]=dis[ now ]+1;

55                 if( vis[ nxt ][ ((dis[ now ]+1)%2) ]==0 ){

56                     vis[ nxt ][ ((dis[ now ]+1)%2) ]=1;

57                     dis[ nxt ]=dis[ now ]+1;

58                     q.push( nxt );

59                 }

60         //    }

61         }

62     }

63 }

64     

65 int main(){

66     int ca;

67     scanf("%d",&ca);

68     for( int tca=1;tca<=ca;tca++ ){

69         int n,m;

70         init();

71         scanf("%d%d",&n,&m);

72         int a,b;

73         while( m-- ){

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

75             addedge( a,b );

76             addedge( b,a );

77             mat[a][b]=mat[b][a]=1;

78         }

79         printf("Case %d: ",tca);

80         if( n<=2 ){

81             printf("Poor JYY.\n");

82             continue;

83         }

84         for( int s=1;s<=n;s++ ){

85             bfs( s,n );

86             //printf("ans:%d\n",ans);

87             if( ans==3 )

88                 break;

89         }

90         if( ans==inf )

91             printf("Poor JYY.\n");

92         else

93             printf("JYY has to use %d balls.\n",ans);

94     }

95     return 0;

96 }