96 lines
1.7 KiB
C++
96 lines
1.7 KiB
C++
#include<stdio.h>
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#include<string.h>
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#include<stdlib.h>
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#include<algorithm>
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#include<iostream>
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#include<vector>
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#include<queue>
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using namespace std;
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#define MAXN 100001
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#define MAXM 300001
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struct people
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{
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int to,next,from;
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};
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people a[MAXM];
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int N,M,last[MAXN],step=0,tot1=0,tot=0,LOW[MAXN],DFN[MAXN],stack[MAXN],ans=0;
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int belong[MAXN],indegree[MAXN],IN,OUT,ans1=0,size1[MAXN],ans2=0;
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bool instack[MAXN];
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void add(int x,int y)
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{
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a[++tot].to=y;
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a[tot].from=x;
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a[tot].next=last[x];
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last[x]=tot;
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}
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void Tarjan(int now)
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{
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DFN[now]=LOW[now]=++step;
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stack[++tot1]=now;
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instack[now]=true;
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for(int i=last[now]; i; i=a[i].next)
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{
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int now_to=a[i].to;
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if(!DFN[now_to])
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{
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Tarjan(now_to);
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LOW[now]=min(LOW[now_to],LOW[now]);
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}
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else if(instack[now_to])
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LOW[now]=min(DFN[now_to],LOW[now]);
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}
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if(DFN[now]==LOW[now])
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{
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int now_out;
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ans++;
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do
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{
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now_out=stack[tot1--];
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instack[now_out]=false;
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belong[now_out]=ans;
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size1[ans]++;
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}
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while(now_out!=now);
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}
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}
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int main()
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{
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// freopen("input.in","r",stdin);
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// freopen("output.out","w",stdout);
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memset(belong,0,sizeof(belong));
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memset(DFN,0,sizeof(DFN));
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memset(LOW,0,sizeof(LOW));
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memset(last,0,sizeof(last));
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memset(instack,false,sizeof(instack));
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scanf("%d%d",&N,&M);
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for(int i=1; i<=M; i++)
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{
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int A,B;
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scanf("%d%d",&A,&B);
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add(A,B);
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}
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for(int i=1; i<=N; i++)
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if(!DFN[i])
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Tarjan(i);
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for(int i=1; i<=tot; i++)
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{
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if(belong[a[i].to]!=belong[a[i].from])
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indegree[belong[a[i].to]]++;
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}
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for(int i=1; i<=ans; i++)
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if(!indegree[i])
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ans1++;
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if(ans1>1)
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for(int i=1; i<=ans; i++)
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if(!indegree[i] && size1[i]==1)
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{
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ans1--;
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break;
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}
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printf("%.6lf\n",1-(double)ans1/N);
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//system("pause");
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return 0;
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}
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