POJ1094 字母排序(拓扑排序)

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<vector>
 5 #include<algorithm>
 6 
 7 using namespace std;
 8 
 9 const int N=5005;
10 vector<int>g[N];
11 int dfn[N],low[N],dg[N],tim;
12 bool vis[N],map[N][N];
13 int n,r;
14 
15 void tarjan(int u,int fa)
16 {
17     dfn[u]=low[u]=++tim;
18     vis[u]=1;
19     for (int i=0;i<g[u].size();i++)
20     {
21         int v=g[u][i]; 
22         if (v==fa) continue;
23         if (!dfn[v])
24         {
25             tarjan(v, u); 
26             low[u]=min(low[u],low[v]);
27         }
28         else if (vis[v]) low[u]=min(low[u],dfn[v]); 
29     }
30 }
31 void init()
32 {
33     memset(dfn,0,sizeof(dfn)); 
34     memset(low,0,sizeof(low)); 
35     memset(vis,0,sizeof(vis)); 
36     tim=0;
37     tarjan(1,-1);
38 }
39 int main()
40 {
41     while (~scanf("%d%d",&n,&r))
42     {
43         for (int i=1;i<=n;i++) 
44             g[i].clear();
45         memset(map,0,sizeof(map));
46         int u,v;
47         for (int i=0;i<r;i++)
48         {
49             scanf("%d%d",&u,&v); 
50             if (!map[u][v])
51             {
52                 g[u].push_back(v);
53                 g[v].push_back(u);
54                 map[u][v]=map[v][u]=1;
55             }
56         }
57 
58         init();
59         memset(dg,0,sizeof(dg));
60         
61         for (int u=1;u<=n;u++)
62             for (int i=0;i<g[u].size();i++)
63             {
64                 int v=g[u][i]; 
65                 if (low[u]!=low[v]) dg[low[u]]++;  
66             }
67 
68         int cnt=0;
69         for (int i=1;i<=n;i++)
70             if (dg[i]==1) cnt++;
71         printf("%d\n",(cnt+1)/2);
72     }
73 }