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;
}