边双连通图

 1 #include<iostream>
 2 #include<algorithm>
 3 #include<cstdio>
 4 #define MAXN 50010
 5 using namespace std;
 6 int n,m,c=2,s=0,d=1,top=1;
 7 int cstack[MAXN],head[MAXN],colour[MAXN],deep[MAXN],low[MAXN],indegree[MAXN];
 8 bool vis[MAXN];
 9 struct Edge{
10     int next,to;
11 }a[MAXN<<1];
12 inline int read(){
13     int date=0,w=1;char c=0;
14     while(c<‘0‘||c>‘9‘){if(c==‘-‘)w=-1;c=getchar();}
15     while(c>=‘0‘&&c<=‘9‘){date=date*10+c-‘0‘;c=getchar();}
16     return date*w;
17 }
18 inline void add(int x,int y){
19     a[c].to=y;a[c].next=head[x];head[x]=c++;
20     a[c].to=x;a[c].next=head[y];head[y]=c++;
21 }
22 void dfs(int x,int edge){
23     deep[x]=low[x]=d++;
24     vis[x]=true;
25     cstack[top++]=x;
26     for(int i=head[x];i;i=a[i].next){
27         int v=a[i].to;
28         if(i==(edge^1))continue;
29         if(!deep[v]){
30             dfs(v,i);
31             low[x]=min(low[x],low[v]);
32         }
33         else if(vis[v])low[x]=min(low[x],deep[v]);
34     }
35     if(low[x]==deep[x]){
36         s++;
37         do{
38             colour[cstack[top-1]]=s;
39             vis[cstack[top-1]]=false;
40         }while(cstack[--top]!=x);
41     }
42 }
43 void work(){
44     int ans=0;
45     for(int i=1;i<=n;i++)ans+=(indegree[i]==1?1:0);
46     printf("%d\n",(ans+1)/2);
47 }
48 void init(){
49     int x,y;
50     n=read();m=read();
51     for(int i=1;i<=m;i++){
52         x=read();y=read();
53         add(x,y);
54     }
55     for(int i=1;i<=n;i++)if(!deep[i])dfs(i,0);
56     for(int i=1;i<=n;i++)
57     for(int j=head[i];j;j=a[j].next){
58         int v=a[j].to;
59         if(colour[i]!=colour[v])indegree[colour[v]]++;
60     }
61 }
62 int main(){
63     init();
64     work();
65     return 0;
66 }