强连通 HDU 1269

n点m边

求是否能从任意a->b b->a

强连通分量等于1

 1 #include<stdio.h>
 2 #include<algorithm>
 3 #include<string.h>
 4 #include<queue>
 5 #include<math.h>
 6 #include<stack>
 7
 8 using namespace std;
 9
10 #define MAXN 100010
11
12 int head[MAXN],low[MAXN],dfn[MAXN],f[MAXN];
13 bool vis[MAXN];
14 int cnt,k,num;
15 struct edg
16 {
17     int fr,to,next;
18
19 }x[MAXN];
20
21 void add(int u,int v)
22 {
23     x[cnt].next=head[u];
24     x[cnt].fr=u;
25     x[cnt].to=v;
26     head[u]=cnt++;
27 }
28 stack<int>s;
29
30 void dfs(int u)
31 {
32     low[u]=dfn[u]=k++;
33     vis[u]=1;
34     s.push(u);
35     int i;
36     for(i=head[u];i!=-1;i=x[i].next)
37     {
38         int t=x[i].to;
39         if(!dfn[t])
40         {
41             dfs(t);
42             low[u]=min(low[u],low[t]);
43         }
44         else if(vis[t])
45             low[u]=min(low[u],dfn[t]);
46     }
47     if(low[u]==dfn[u])
48     {
49         num++;
50         while(!s.empty())
51         {
52             int now=s.top();
53             s.pop();
54             vis[now]=0;
55             f[now]=num;
56             if(now==u)break;
57         }
58     }
59 }
60 int main()
61 {
62     int n,m;
63
64     while(scanf("%d%d",&n,&m)!=EOF)
65     {
66         if(n+m==0)
67             break;
68         int i;
69         memset(head,-1,sizeof(head));
70         cnt=0;
71         for(i=1;i<=m;i++)
72         {
73             int a,b;
74             scanf("%d%d",&a,&b);
75             add(a,b);
76         }
77         k=1;
78         num=0;
79         memset(dfn,0,sizeof(dfn));
80         memset(low,0,sizeof(low));
81         memset(vis,0,sizeof(vis));
82         memset(f,0,sizeof(f));
83         for(i=1;i<=n;i++)
84         {
85             if(!dfn[i])
86                 dfs(i);
87         }
88         if(num==1)
89             printf("Yes\n");
90         else
91             printf("No\n");
92     }
93     return 0;
94 }