HDU4463 Outlets

　　原题传送：http://acm.hdu.edu.cn/showproblem.php?pid=4463

　　这题算是比较裸的最小生成树。由于耐克店和苹果店要有公路相连，所以要先加上这条公路长度，然后将这条公路长度置0，那么就不会影响求最小生成树的贪心过程。

#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <math.h>
const int maxn = 50 + 2;

struct node 
{
    double x, y;
}s[maxn];
int u[maxn*maxn], v[maxn*maxn], p[maxn*maxn], r[maxn*maxn], a, b, n, m;
double w[maxn*maxn];

int cmp(const int i, const int j)
{return w[i] - w[j] < 1e-8;}
 
int find(int x)
{return p[x] == x ? x : p[x] = find(p[x]);}

double len(int i, int j)
{return sqrt((s[j].y - s[i].y) * (s[j].y - s[i].y) + (s[j].x - s[i].x) * (s[j].x - s[i].x));}

double Kruskal()
{
    double mst = 0;
    mst += len(a-1, b-1);
    for(int i = 0; i < n; i ++)
        p[i] = i;
    for(int i = 0; i < m; i ++)
        r[i] = i;
    std::sort(r, r + m, cmp);
    for(int i = 0; i < m; i ++)
    {
        int e = r[i];
        int x = find(u[e]);
        int y = find(v[e]);
        if(x != y)
        {
            mst += w[e];
            p[x] = y;
        }
    }
    return mst;
}

int main()
{
    while(scanf("%d", &n) == 1 && n)
    {
        scanf("%d%d", &a, &b);
        if(a > b) std::swap(a, b);
        for(int i = 0; i < n; i ++)
            scanf("%lf%lf", &s[i].x, &s[i].y);
        m = 0;
        for(int i = 0; i < n; i ++)
            for(int j = i + 1 ; j < n; j ++)
                u[m] = i, v[m] = j, w[m++] = (i == a-1 && j == b-1) ? 0.0 : len(i, j);
        
        printf("%.2f\n", Kruskal());
    }
    return 0;
}