Problem C:

Nicholas Y. Alford was a cat lover. He had a garden in a village and kept many cats in his garden. The cats were so cute that people in the village also loved them.

One day, an evil witch visited the village. She envied the cats for being loved by everyone. She drove magical piles in his garden and enclosed the cats with magical fences running between the piles. She said “Your cats are shut away in the fences until they become ugly old cats.” like a curse and went away.

Nicholas tried to break the fences with a hummer, but the fences are impregnable against his effort. He went to a church and asked a priest help. The priest looked for how to destroy the magical fences in books and found they could be destroyed by holy water. The Required amount of the holy water to destroy a fence was proportional to the length of the fence. The holy water was, however, fairly expensive. So he decided to buy exactly the minimum amount of the holy water required to save all his cats. How much holy water would be required?

Input

The input has the following format:

N M
x1 y1
.
.
.
xN yN
p1 q1
.
.
.
pM qM

The first line of the input contains two integers N (2 ≤ N ≤ 10000) and M (1 ≤ M). N indicates the number of magical piles and M indicates the number of magical fences. The following N lines describe the coordinates of the piles. Each line contains two integers xi and yi (-10000 ≤ xi, yi ≤ 10000). The following M lines describe the both ends of the fences. Each line contains two integers pj and qj (1 ≤ pj, qj ≤ N). It indicates a fence runs between the pj-th pile and the qj-th pile.

You can assume the following:

No Piles have the same coordinates.
A pile doesn’t lie on the middle of fence.
No Fences cross each other.
There is at least one cat in each enclosed area.
It is impossible to destroy a fence partially.
A unit of holy water is required to destroy a unit length of magical fence.

Output

Output a line containing the minimum amount of the holy water required to save all his cats. Your program may output an arbitrary number of digits after the decimal point. However, the absolute error should be 0.001 or less.

Sample Input 1

Output for the Sample Input 1

3.000

Sample Input 2

Output for the Sample Input 2

0.000

Sample Input 3

Output for the Sample Input 3

7.236

Sample Input 4

Output for the Sample Input 4

31.000

题意大致是给一些点和一些边问怎么样用最小的代价去掉一些边来使图中无回路

可以通过求最大生成树的长度然后总长减去最大生成树的长度就是所求结果。

代码如下：

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <math.h>
using namespace std;
const int maxn=10005;
int n,m;
struct loc
{
    int x,y;
};
loc l[maxn];
struct edge
{
    int s,e;
    double len;
};
edge e[maxn<<2];
int a[maxn];
double total;
double dis (int x1,int y1,int x2,int y2)
{
    return sqrt((double)((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)));
}
int compare (edge a,edge b)
{
    return a.len>b.len;
}
void init ()
{
    for (int i=1;i<=n;i++)
         a[i]=i;
}
int finds(int x)
{
    if(x==a[x])
        return x;
    return a[x]=finds(a[x]);
}
bool unit (int x,int y)
{
    int temp1=finds(x);
    int temp2=finds(y);
    if(temp1!=temp2)
    {
        a[temp1]=temp2;
        return true;
    }
    return false;
}
void kual ()
{
    sort (e,e+m,compare);
    int num=n;
    double sum=0;
    for (int i=0;i<m&&num>1;i++)
    {
        if(unit(e[i].s,e[i].e))
        {
            sum+=e[i].len;
            num--;
        }
    }
    printf("%.3lf\n",total-sum);
}
int main()
{
    total=0;
    scanf("%d%d",&n,&m);
    init();
    for (int i=1;i<=n;i++)
        scanf("%d%d",&l[i].x,&l[i].y);
    for (int i=0;i<m;i++)
    {
        int x,y;
       scanf("%d%d",&x,&y);
       e[i].s=x;
       e[i].e=y;
       e[i].len=dis(l[x].x,l[x].y,l[y].x,l[y].y);
       total+=e[i].len;
    }
    kual();
    return 0;
}

AOJ 2224 Save your cats