poj3155 最大密度子图

求最大密度子图

记得在最后一次寻找的时候记得将进入的边放大那么一点点，这样有利于当每条边都满流的情况下会选择点

#include <iostream>
#include <algorithm>
#include <string.h>
#include <cstdio>
#include <vector>
#include <queue>
#include <cmath>
using namespace std;
const int maxn=;
const double eps=0.00000001;
const double INF=*;
vector<int>ans;
struct Dinic
{
     struct Edge
     {
         int from,to;
         double cap,flow;
         Edge(int cfrom=,int cto=,double ccap=,double cflow=)
         {
             from=cfrom; to=cto; cap=ccap; flow=cflow;
         }
     };
     int n,m,s,t;
     vector<Edge>edges;
     vector<int>G[maxn];
     bool vis[maxn];
     int d[maxn];
     int cur[maxn];
     void init(int n)
     {
         this->n=n;
         m=;
         edges.clear();
         for(int i=; i<=n; i++)G[i].clear();
     }
     void AddEdge(int from,int to,double cap)
     {
           edges.push_back(Edge(from,to,cap,));
           edges.push_back(Edge(to,from,,));
           m+=;
           G[from].push_back(m-);
           G[to].push_back(m-);
     }
     bool BFS()
     {
         memset(vis,false,sizeof(vis));
         queue<int>Q;
         Q.push(s);
         d[s]=;
         vis[s]=;
         while(!Q.empty())
         {
             int x=Q.front(); Q.pop();
             for(int i=; i<G[x].size(); i++)
             {
                 Edge &e =edges[G[x][i]];
                 if(vis[e.to]==false&&e.cap>e.flow)
                 {
                     vis[e.to]=;
                     d[e.to]=d[x]+;
                     Q.push(e.to);
                 }
             }
         }
         return vis[t];
     }
     double DFS(int x, double a)
     {
         if(x==t||a==)return a;
        double flow=,f;
        for(int &i=cur[x]; i<G[x].size(); i++)
        {
            Edge &e=edges[G[x][i]];
            if(d[x]+==d[e.to]&&(f=DFS(e.to,min(a,e.cap-e.flow)))>)
            {
                e.flow+=f;
                edges[G[x][i]^].flow-=f;
                flow+=f;
                a-=f;
                if(a==)break;
            }
        }
        return flow;
     }
     double Maxflow(int s,int t)
     {
         this->s=s; this->t=t;
         double flow=;
         while(BFS())
         {
             memset(cur,,sizeof(cur));
             flow+=DFS(s,INF);
         }
         return flow;
     }
     void finde(int x)
     {
         vis[x]=;
         ans.push_back(x);
         for(int i=;i<G[x].size(); i++)
         {
             Edge &e=edges[G[x][i]];
             if(vis[e.to]||e.cap<=e.flow)continue;
             finde(e.to);
         }
     }
     void solve()
     {
         memset(vis,false,sizeof(vis));
         vis[s]=;
         vis[t]=;
         for(int i=; i<G[s].size(); i++)
         {
             Edge &e=edges[G[s][i]];
             if(vis[e.to]||e.cap<=e.flow)continue;
                finde(e.to);
         }
     }
}T;
int d[maxn];
int A[],B[];
double U;
void build(int n,int m,double g,int op=)
{
    T.init(n+);
    for(int i=; i<=n; i++)
    {
        T.AddEdge(n+,i,U+op*eps*);
        T.AddEdge(i,n+,U+g*-d[i]);
    }
    for(int i=; i<=m; i++)
    {
        T.AddEdge(A[i],B[i],);
        T.AddEdge(B[i],A[i],);
    }
}
int main()
{
    int n,m;
    while(scanf("%d%d",&n,&m)==)
    {
        if(m==)
        {
            printf("1\n1\n");continue;
        }
        memset(d,,sizeof(d));
        for(int i=; i<=m; i++)
        {
            scanf("%d%d",&A[i],&B[i]);
            d[A[i]]++;d[B[i]]++;
        }
        U=m;
        double cL=0.0,cR=1.0*m;
        for(int i=; i<; i++)
        {
            double mid=(cL+cR)/;
            build(n,m,mid);
            double ans=T.Maxflow(n+,n+);
            ans=(U*n-ans)/;
            if(ans>) cL=mid;
            else cR=mid;
        }
        build(n,m,cL,);
        T.Maxflow(n+,n+);
        ans.clear();
        T.solve();
        sort(ans.begin(),ans.end());
        int ge=unique(ans.begin(),ans.end())-ans.begin();
        printf("%d\n",ge);
        for(int i=; i<ge;i++)
            printf("%d\n",ans[i]);
    }
    return ;
}