pku 2031 Building a Space Station 最小生成树+精度控制

http://poj.org/problem?id=2031

题意就是给你n个球的（球心以及半径）,然后求最小生成树就欧了，这里的关键部分是对于重复的球的路径长度的处理

#include <iostream>

#include <cstring>

#include <cstdio>

#include <cmath>

#define maxn 107

#define inf 999999999

using namespace std;

const double eps = 1e-6;

struct node

{

    double x,y,z,r;

}p[maxn];

double map[100][100],dis[maxn];

bool vt[maxn];

int n;

int cmp(double x)

{

    if (x > eps) return 1;

    else if (x < -eps) return -1;

    else return 0;

}

double getlens(int i,int j)

{

    double x = p[i].x - p[j].x;

    double y = p[i].y - p[j].y;

    double z = p[i].z - p[j].z;

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

    if (cmp(d) < 0) return 0;//如果有重复的出现，返回0

    else return d;

}

void prim()

{

    int i,j,k;

    double min;

    double ans = 0;

    for (i = 0; i < n; ++i)

    {

        vt[i] = false;

        dis[i] = map[0][i];

    }

    vt[0] = true;

    for (k = 1; k < n; ++k)

    {

        j = 0; min = inf;

        for (i = 1; i < n; ++i)

        {

            if (!vt[i] && min > dis[i])

            {

                j = i; min = dis[i];

            }

        }

        vt[j] = true;

        ans += min;

        for (i = 1; i < n; ++i)

        {

            if (!vt[i])

            {

                if (dis[i] > map[i][j])

                dis[i] = map[i][j];

            }

        }

    }

    printf("%.3lf\n",ans);

}

int main()

{

    int i,j;

    while (~scanf("%d",&n))

    {

        if (!n) break;

        for (i = 0; i < n; ++i)

        scanf("%lf%lf%lf%lf",&p[i].x,&p[i].y,&p[i].z,&p[i].r);

        for (i = 0; i < n; ++i)

        {

            for (j = 0; j < n; ++j)

            {

                map[i][j] = (i != j)*inf;

            }

        }

        /*for (i = 0; i < n; ++i)

        {

            for (j = 0; j < n; ++j)

            {

               printf("%.3lf ",map[i][j]);

            }

            printf("\n");

        }*/

        for (i = 0; i < n; ++i)

        {

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

            {

                map[i][j] = map[j][i] = getlens(i,j);

            }

        }

        prim();

    }

    return 0;

}