poj1292

prim，把每个墙看成一个节点，从起点用prim求最小生成树，直到覆盖到终点为止，输出最小生成树中的最大边

#include <cstdio>

#include <cmath>

#include <cstdlib>

#include <algorithm>

#include <cstring>

using namespace std;



#define MAX_WALL_NUM 1005

#define INF 0x3f3f3f3f



struct Wall

{

    int s, e, pos;

    bool h;        //direction: is horizental

}wall[MAX_WALL_NUM];



int wall_num;

double graph[MAX_WALL_NUM][MAX_WALL_NUM];



void input()

{

    for (int i = 0; i < wall_num; i++)

    {

        int x, y, l;

        scanf("%d%d%d", &x, &y, &l);

        if (l < 0)

        {

            wall[i].h = false;

            swap(x, y);

        }else

            wall[i].h = true;

        wall[i].s = x;

        wall[i].e = x + abs(l);

        wall[i].pos = y;

    }

}



bool overlap(Wall a, Wall b)

{

    if (a.s > b.e || a.e < b.s)

        return false;

    return true;

}



double cal(Wall a, Wall b)

{

    if (a.h == b.h)

    {

        int dist1 = abs(a.pos - b.pos);

        if (overlap(a, b))

            return dist1;

        else

        {

            int dist2 = min(abs(a.s - b.e), abs(a.e - b.s));

            return sqrt(dist1 * dist1 + dist2 * dist2);

        }

    }

    if (a.s <= b.pos && a.e >= b.pos && b.s <= a.pos && b.e >= a.pos)

        return 0;

    if (a.s <= b.pos && a.e >= b.pos)

        return min(abs(b.s - a.pos), abs(b.e - a.pos));

    if (b.s <= a.pos && b.e >= a.pos)

        return min(abs(a.s - b.pos), abs(a.e - b.pos));

    int dist1 = min(abs(a.s - b.pos), abs(a.e - b.pos));

    int dist2 = min(abs(b.s - a.pos), abs(b.e - a.pos));

    return sqrt(dist1 * dist1 + dist2 * dist2);

}



void calculate_graph()

{

    for (int i = 0; i < wall_num; i++)

    {

        for (int j = i + 1; j < wall_num; j++)

        {

            graph[i][j] = graph[j][i] = cal(wall[i], wall[j]);

        }

    }

}



double prim()

{

    bool vis[MAX_WALL_NUM];

    double dist[MAX_WALL_NUM];

    memset(vis, 0, sizeof(vis));

    fill(dist, dist + wall_num, INF);

    dist[0] = 0;

    double ret = 0;

    while (!vis[1])

    {

        double min_dist = INF;

        int temp = -1;

        for (int i = 0; i < wall_num; i++)

            if (!vis[i] && dist[i] < min_dist)

            {

                temp = i;

                min_dist = dist[i];

            }

        if (temp == -1)

            return -1;

        vis[temp] = true;

        dist[temp] = 0;

        ret = max(ret, min_dist);

        for (int i = 0; i < wall_num; i++)

            dist[i] = min(dist[i], dist[temp] + graph[temp][i]);

    }

    return ret;

}



int main()

{

    while (scanf("%d", &wall_num), wall_num)

    {

        input();

        calculate_graph();

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

    }

    return 0;

}

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