157 lines
2.2 KiB
C++
157 lines
2.2 KiB
C++
#include<cstdio>
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#include<string.h>
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#include<algorithm>
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#include<vector>
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using namespace std;
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const int N=1e5+5,NN=3e5+5;
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int n,e;
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int to[NN],ne[NN];
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int h[N];
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int dfn[N],low[N];
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bool b[N];
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int s[N];//stack
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int cnt;
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int tot,top;
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int id;
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int nid[N];
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int now;
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bool no[N];
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int fa[N];
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vector<int> tfa[N];
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void add(int u,int v)
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{
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to[++cnt]=v;
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ne[cnt]=h[u];
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h[u]=cnt;
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}
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void initialise(int n)
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{
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cnt=0;
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tot=0;
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top=0;
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id=n;
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memset(b,0,sizeof(bool)*(n+1));
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memset(no,0,sizeof(bool)*(n+1));
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memset(h,0,sizeof(int)*(n+1));
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memset(dfn,0,sizeof(int)*(n+1));//WA 根据它判断有无经过
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}
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void tarjan(int u)
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{
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// printf("%d:",u);
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int v;
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dfn[u]=++tot;
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low[u]=tot;
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s[++top]=u;
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b[u]=1;
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for(int i=h[u]; i; i=ne[i])
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{
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v=to[i];
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// printf("%d ",v);
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if(!dfn[v])
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{
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tarjan(v);
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low[u]=low[u]<low[v]? low[u]:low[v];
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}
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else if(b[v])
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{
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low[u]=low[u]<dfn[v]? low[u]:dfn[v];
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}
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}
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if(low[u]==dfn[u])
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{
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id++;
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// printf("(%d):",id);
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do
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{
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// printf("%d ",s[top]);
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nid[s[top]]=id;
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b[s[top]]=0;
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}
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while(s[top--]!=u&&top>0);
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// printf("\n");
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}
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}
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void dfs(int u,int fat)
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{
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for(int i=h[u]; i; i=ne[i])
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{
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int v=to[i];
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tfa[v].push_back(fat);
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dfs(v,fat);//这句居然没写
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}
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}
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int main()
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{
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int u,v;
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// while(~scanf("%d",&n)&&n)
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// {
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scanf("%d",&n);
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initialise(n);
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scanf("%d",&e);
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for(int i=1; i<=e; i++)
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{
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scanf("%d%d",&u,&v);
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add(u,v);
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}
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//一开始放在前面，忽略了没有边的情况
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for(int i=1; i<=n; i++)
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{
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// add(0,i);
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if(!dfn[i])
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{
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tarjan(i);
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}
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}
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// tarjan(0);
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int l=cnt+1;
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for(int i=1; i<=n; i++)
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{
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for(int j=h[i]; j; j=ne[j])
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{
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v=to[j];
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if(nid[v]!=nid[i])
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{
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add(nid[i],nid[v]);
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fa[nid[v]]++;
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// printf("(%d)to(%d)%d\n",nid[i],nid[v],fa[nid[v]]);
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}
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}
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}
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int ans=0;
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for(int i=n+1; i<=id; i++)
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{
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// printf("(%d)",i);
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if(fa[i]==0 )
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{
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ans++;
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dfs(i,i);
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}
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}
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// printf("%d ",ans);
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bool des=0;
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for(int i=n+1; i<=id; i++)
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{
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if(fa[i]==0 )
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{
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int j=h[i];//i ->u
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for(; j; j=ne[j])
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{
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v=to[j];
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// printf("%d to %d:%d\n",i,v,tfa[v].size());
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if(tfa[v].size()==1)
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{
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break;
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}
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}
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if(!j)
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{
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des=1;
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break;
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}
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}
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}
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printf("%.6lf",1.0-(ans-des)*1.0/n);
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// }
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return 0;
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}
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