Codefoces 277 E. Binary Tree on Plane

题目链接：http://codeforces.com/problemset/problem/277/E

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参考了这篇题解：http://blog.csdn.net/Sakai_Masato/article/details/50775315

没看出来是费用流啊...我好菜啊。

我写得有一点和上面这篇题解不同：在判断是否无解时我直接记录最大流，判断最大流是否等于$n-1$(似乎是等价的...)

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<vector>
#include<cstdlib>
#include<cmath>
#include<cstring>
using namespace std;
#define maxn 1010
#define inf 0x7fffffff
#define llg int
#define sqr(_) ((_)*(_))
#define yyj(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
llg n,m;

struct node 
{
    llg u,v,c,next;
    double w;
};

struct FLOW
{
    llg cnt,maxflow,S,T,pre[maxn],N;
    llg head[maxn],dl[maxn*maxn];
    bool bj[maxn];
    double mincost,dis[maxn];
    node e[maxn*maxn];


    void init()
    {
        memset(head,-1,sizeof(head));
        mincost=cnt=maxflow=0;
        maxflow=0; mincost=0;
    }

    void link(llg u,llg v,double w,llg c)
    {
        e[cnt].u=u,e[cnt].v=v,e[cnt].w=w,e[cnt].c=c;
        e[cnt].next=head[u],head[u]=cnt++;
        
        e[cnt].u=v,e[cnt].v=u,e[cnt].w=-w,e[cnt].c=0;
        e[cnt].next=head[v],head[v]=cnt++;
    }

    void updata()
    {
        llg f=inf;
        for (llg i=T;i!=S;i=e[pre[i]].u) f=min(f,e[pre[i]].c);
        maxflow+=f;
        for (llg i=T;i!=S;i=e[pre[i]].u)
        {
            e[pre[i]].c-=f;
            e[pre[i]^1].c+=f;
            mincost+=(double)f*e[pre[i]].w;
        }
    }

    bool spfa()
    {
        llg u,v;
        double w;
        memset(pre,-1,sizeof(pre));
        memset(bj,0,sizeof(bj));
        for (llg i=0;i<=N;i++) dis[i]=inf;
        llg l=0,r=1;
        dl[r]=S; bj[S]=1; dis[S]=0;
        do
        {
            bj[u=dl[++l]]=0;
            for (llg i=head[u];i!=-1;i=e[i].next)
            {
                v=e[i].v,w=e[i].w;
                if (e[i].c && dis[v]>dis[u]+w)
                {
                    dis[v]=dis[u]+w;
                    pre[v]=i;
                    if (!bj[v]) bj[v]=1,dl[++r]=v;
                }
            }
        }while (l<r);
    
        if (pre[T]==-1) return 0;
        return 1;
    }
    
    void work()
    {
        while (spfa()) 
            updata();
    }

}G;

inline int getint()
{
    int w=0,q=0; char c=getchar();
    while((c<'0' || c>'9') && c!='-') c=getchar(); if(c=='-') q=1,c=getchar(); 
    while (c>='0' && c<='9') w=w*10+c-'0', c=getchar(); return q ? -w : w;
}

struct POINT{double x,y;}po[maxn];

bool cmp(const POINT&x,const POINT&y) {return x.y==y.y?x.x<y.x:x.y<y.y;}

double dis(const POINT&x,const POINT&y) {return sqrt(sqr(x.x-y.x)+sqr(x.y-y.y));}

int main()
{
    yyj("flow");
    cin>>n;
    G.init();
    G.N=n*2+10;
    for (llg i=1;i<=n;i++) scanf("%lf%lf",&po[i].x,&po[i].y);
    sort(po+1,po+n+1,cmp);
    reverse(po+1,po+1+n);
    for (llg i=1;i<=n;i++)
        for (llg j=i+1;j<=n;j++)
            if (po[i].y>po[j].y)
                G.link(i,j+n,dis(po[i],po[j]),(llg)1);
    G.S=2*n+1,G.T=2*n+2;
    for (llg i=1;i<=n;i++)
    {
        G.link(G.S,i,0,(llg)2);
        G.link(i+n,G.T,0,(llg)1);
    }
    G.work();
    if (G.maxflow!=n-1) {cout<<-1;} else printf("%.9lf",G.mincost);
    return 0;
}