uva 610(tarjan的应用)

题目链接：http://acm.hust.edu.cn/vjudge/problem/viewProblem.action?id=23727

思路：首先是Tarjan找桥，对于桥，只能是双向边，而对于同一个连通分量而言，只要重新定向为同一个方向即可。

 1 #include<iostream>

 2 #include<cstdio>

 3 #include<cstring>

 4 #include<algorithm>

 5 #include<stack>

 6 #include<queue>

 7 #include<vector>

 8 using namespace std;

 9 #define MAXN 1111

10 typedef pair<int,int>PP;

11 

12 int low[MAXN],dfn[MAXN];

13 bool mark[MAXN];

14 int _count,cnt;

15 vector<int>g[MAXN];

16 vector<PP>bridge;

17 bool vis[MAXN][MAXN];

18 int n,m;

19 

20 void Tarjan(int u,int father)

21 {

22     int flag=0;

23     low[u]=dfn[u]=++cnt;

24     mark[u]=true;

25     for(int i=0;i<(int)g[u].size();i++){

26         int v=g[u][i];

27         if(v==father&&!flag){ flag=1;continue; }

28         if(dfn[v]==0){

29             Tarjan(v,u);

30             low[u]=min(low[u],low[v]);

31             if(low[v]>dfn[u]){

32                 bridge.push_back(make_pair(u,v));

33                 bridge.push_back(make_pair(v,u));

34                 vis[u][v]=vis[v][u]=true;

35             }else {

36                 bridge.push_back(make_pair(u,v));

37                 vis[u][v]=vis[v][u]=true;

38             }    

39         }else if(mark[v]){

40             low[u]=min(low[u],dfn[v]);

41             if(!vis[u][v]){

42                 bridge.push_back(make_pair(u,v));

43                 vis[u][v]=vis[v][u]=true;

44             }

45         }

46     }

47 }

48 

49 int main()

50 {

51     int u,v,t=1;

52     while(~scanf("%d%d",&n,&m)){

53         if(n==0&&m==0)break;

54         for(int i=0;i<=n+1;i++)g[i].clear();

55         bridge.clear();

56         while(m--){

57             scanf("%d%d",&u,&v);

58             g[u].push_back(v);

59             g[v].push_back(u);

60         }

61         memset(mark,false,sizeof(mark));

62         memset(dfn,0,sizeof(dfn));

63         memset(vis,false,sizeof(vis));

64         _count=cnt=0;

65         Tarjan(1,-1);

66         printf("%d\n\n",t++);

67         for(int i=0;i<(int)bridge.size();i++){

68             printf("%d %d\n",bridge[i].first,bridge[i].second);

69         }

70         puts("#");

71     }

72     return 0;

73 }

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