依然是最小生成樹問題，使用並查集＋kruskal實現。不贅述。

題目描述：

    In an episode of the Dick Van Dyke show, little Richie connects the freckles on his Dad's back to form a picture of the Liberty Bell. Alas, one of the freckles turns out to be a scar, so his Ripley's engagement falls through. 
    Consider Dick's back to be a plane with freckles at various (x,y) locations. Your job is to tell Richie how to connect the dots so as to minimize the amount of ink used. Richie connects the dots by drawing straight lines between pairs, possibly lifting the pen between lines. When Richie is done there must be a sequence of connected lines from any freckle to any other freckle. 

輸入：

    The first line contains 0 < n <= 100, the number of freckles on Dick's back. For each freckle, a line follows; each following line contains two real numbers indicating the (x,y) coordinates of the freckle.

輸出：

    Your program prints a single real number to two decimal places: the minimum total length of ink lines that can connect all the freckles.

樣例輸入：

3
1.0 1.0
2.0 2.0
2.0 4.0

樣例輸出：

3.41

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#define MAXSIZE 100 
using namespace std;
struct Edge{
    int start,end;
    double weight;
    Edge(){
    }
    Edge(int start,int end,double weight){
        this->start=start;
        this->end=end;
        this->weight=weight;
    }
}; 
bool cmp(Edge a,Edge b){
    return a.weight<b.weight;
}
struct Vex{
    int root;
    double x,y;
    Vex(double x,double y){
        this->x=x;
        this->y=y;
        this->root=-1;
    }
};
struct Set{ //UnionFindSet ´Ó0¿ªÊ¼ 
    int setSize;
    vector<Edge> edge;
    vector<Vex> vex;
    int initSet(int setSize){
        this->setSize=setSize;
        edge.clear();
        vex.clear();
    }
    int findRoot(int x){
        if (vex[x].root==-1)
            return x;
        else
            return vex[x].root=findRoot(vex[x].root);
    }
    int unionSet(int x,int y){
        int xroot=findRoot(x);
        int yroot=findRoot(y);
        if(xroot!=yroot){//if x&y are not in the same set ÄÇô˵Ã÷ÆäÖÐÒ»¸ö»¹Êdzõ̬£¬Ò»´Î¶¼Ã»Óв¢¹ý
            vex[xroot].root=yroot; //merge xset into yset 
            return yroot;
        } 
        return -1;
    }
};
int main(){
    int n;
    double x,y,weight,totalWeight;
    Set set;
    while (cin>>n){
        //initiate
        set.initSet(n);
        totalWeight=0;
        //input
        for (int i=0;i<n;i++){
            cin>>x>>y;
            set.vex.push_back(Vex(x,y));
            for (int j=0;j<i;j++){
                set.edge.push_back(Edge(i,j,sqrt(pow(x-set.vex[j].x,2)+pow(y-set.vex[j].y,2))));
            }
        }
        //sort
        sort(&set.edge[0],&set.edge[0]+set.edge.size(),cmp);
        //kruskal
        for (int i=0;i<set.edge.size();i++){
            if (set.unionSet(set.edge[i].start,set.edge[i].end)!=-1){
                totalWeight+=set.edge[i].weight;
            }
        }
        //output
        cout<<setiosflags(ios::fixed)<<setprecision(2)<<totalWeight<<endl;
    }
    return true;
}
 
/**************************************************************
    Problem: 1144
    User: bit3125
    Language: C++
    Result: Accepted
    Time:10 ms
    Memory:1668 kb
****************************************************************/