hdu 4463 有一条边必须加上 （2012杭州区域赛K题）

 # include <iostream>

 # include <cstdio>

 # include <cstring>

 # include <algorithm>

 # include <cmath>

 # define LL long long

 using namespace std ;

 const int INF=0x3f3f3f3f;

 const int MAXN=;

 bool vis[MAXN];

 double lowc[MAXN];

 int n ;

 double cost[MAXN][MAXN] ;

 struct poin

 {

     int x ;

     int y ;

 }p[MAXN];

 double Prim()//点是0~n-1

 {

     double ans=;

     memset(vis,false,sizeof(vis));

     vis[]=true;

     for(int i=;i<n;i++)lowc[i]=cost[][i];

     for(int i=;i<n;i++)

     {

         double minc=INF;

         int p=-;

         for(int j=;j<n;j++)

             if(!vis[j]&&minc>lowc[j])

             {

                 minc=lowc[j];

                 p=j;

             }

             if(minc==INF)return -;//原图不连通

             ans+=minc;

             vis[p]=true;

             for(int j=;j<n;j++)

                 if(!vis[j]&&lowc[j]>cost[p][j])

                     lowc[j]=cost[p][j];

     }

     return ans;

 }

 int main()

 {

    // freopen("in.txt","r",stdin) ;

     while(scanf("%d" , &n) != EOF)

     {

         if (n == )

             break ;

         int i , j ;

         int pp , qq ;

         scanf("%d %d" , &pp , &qq) ;

         for (i =  ; i < n ; i++)

             scanf("%d %d" , &p[i].x , &p[i].y) ;

         for (i =  ; i < n ; i++)

             for (j = i+ ; j < n ; j++)

             {

                 double t = sqrt((double)(p[i].x - p[j].x) * (p[i].x - p[j].x) + (p[i].y - p[j].y) * (p[i].y - p[j].y)) ;

                 cost[i][j] = t ;

                 cost[j][i] = t ;

             }

         double k = cost[pp-][qq-] ;

         cost[pp-][qq-] =  ;

         cost[qq-][pp-] =  ;

         k += Prim() ;

         printf("%.2lf\n" , k) ;

     }

     return  ;

 }