2014西安赛区C题

将A[i]同他后面比他小的建边，然后求最大密度子图

#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;
     }
}T;
int d[maxn];
int A[maxn*maxn],B[maxn*maxn];
int N[maxn];
double U;
void build(int n,int m,double g)
{
    T.init(n+);
    for(int i=; i<=n; i++)
    {
        T.AddEdge(n+,i,U);
        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 cas;
    scanf("%d",&cas);
    for(int cc=; cc<=cas; cc++)
    {
        int n;
        scanf("%d",&n);
        for(int i=; i<=n; i++)
            scanf("%d",&N[i]);
        int m=;
        memset(d,,sizeof(d));
        for(int i=; i<=n; i++)
            for(int j=i+; j<=n; j++)
           if(N[i]>N[j]){
                d[N[i]]++;d[N[j]]++;
             A[m]=N[i];B[m]=N[j]; m++;
          }
        if(m==)
            {
                printf("Case #%d: 0.0000000\n",cc);
                continue;
            }
        double L=,R=m;
        U=m;
        while(R-L>=eps)
        {
            double mid=(L+R)*0.5;
            build(n,m,mid);
            double ans=T.Maxflow(n+,n+);
            ans=(U*n-ans)*0.5;
            if(ans>)L=mid;
            else R=mid;
        }
        printf("Case #%d: %.7lf\n",cc,L );
    }
    return ;
}
/*
2
5
3 4 2 5 1
5
2 1 3 4 5
*/