hdu4463Prim简单变形

最小生成树Prim算法的简单变形，Prim算法就是贪心的思想，从任一点开始，依次选择离已知点最近的点扩展，那么这题只要初始的时候把给出的两点加到已知点中即可。

/*

 * hdu4463/win.cpp

 * Created on: 2012-11-17

 * Author    : ben

 */

#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <cmath>

#include <ctime>

#include <iostream>

#include <algorithm>

#include <queue>

#include <set>

#include <map>

#include <stack>

#include <string>

#include <vector>

#include <deque>

#include <list>

#include <functional>

#include <numeric>

#include <cctype>

using namespace std;

/**

 * 普里姆算法求最小生成树。map存储图的邻接矩阵。

 * N为图的顶点数

 * 返回最小生成树的权值，返回-1表示图不连通

 * typec可以是int、float、double等类型

 */

typedef double typec;

const int MAXN = 60;

const typec INF = 0x7fffffff;

typec graph[MAXN][MAXN];

int N, p, q;

typec MST_Prim() {

    typec lowcost[MAXN];

    int i, j, k;

    typec minc, ret = 0;

    copy(graph[p], graph[p] + N, lowcost);

    lowcost[q] = lowcost[p] = -INF;

    ret += graph[p][q];

    for(int j = 0; j < N; j++) {

        if(lowcost[j] > graph[q][j]) {

            lowcost[j] = graph[q][j];

        }

    }

    for(i = 2; i < N; i++) {

        minc = INF;

        for(j = 0; j < N; j++) {

            if(lowcost[j] > -INF && lowcost[j] < minc) {

                minc = lowcost[j];

                k = j;

            }

        }

        if(minc == INF) {

            return -1;

        }

        ret += minc;

        lowcost[k] = -INF;

        for(j = 0; j < N; j++) {

            if(lowcost[j] > graph[k][j]) {

                lowcost[j] = graph[k][j];

            }

        }

    }

    return ret;

}



typedef pair<int, int> MyPoint;



inline double mydis(const MyPoint &p1, const MyPoint &p2) {

    return sqrt((p1.first - p2.first) * (p1.first - p2.first) + (p1.second - p2.second) * (p1.second - p2.second));

}



void build_graph() {

    int a, b;

    scanf("%d%d", &p, &q);

    p--, q--;

    vector<pair<int, int> > g;

    for(int i = 0; i < N; i++) {

        scanf("%d%d", &a, &b);

        g.push_back(make_pair(a, b));

    }

    for(int i = 0; i < N; i++) {

        graph[i][i] = 0;

        for(int j = i + 1; j < N; j++) {

            graph[i][j] = graph[j][i] = mydis(g[i], g[j]);

        }

    }

}



int main() {

#ifndef ONLINE_JUDGE

    freopen("data.in", "r", stdin);

#endif

    while(scanf("%d", &N) == 1 && N > 0) {

        build_graph();

        printf("%.2f\n", MST_Prim());

    }

    return 0;

}