判断是否共线用map记录下斜率；

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>
#include<algorithm>
#include<math.h>
#include<map>
using namespace std;
#define N 1022
const int INF = 1<<30-1;
bool vis[2020];
int mat[1022][1022],lowcost[1022],pre[1033];
double cost[1022][1022];
int maxlong,star,end;
double mathh(int a,int c,int b,int d)
{
    return sqrt((a-b)*(a-b)*1.0+(c-d)*(c-d)*1.0);
}
double xielv(int a,int c,int b,int d)
{
    if(a==b)//垂直x轴
        return INF;
    else return (d-c)*1.0/(b-a);
}
struct node
{
    int x,y;
} point[1022];
bool cmp(node a,node b)
{
    if(a.x==b.x)
        return a.y<b.y;
    return a.x<b.x;
}
struct node1
{
    int star,step;
};
void Dijkstra(int n,int beg)
{
    for(int i=0; i<n; i++)
    {
        lowcost[i]=INF;
        vis[i]=false;
        pre[i]=-1;
    }
    lowcost[beg]=0;
    for(int j=0; j<n; j++)
    {
        int k=-1;
        int Min=INF;
        for(int i=0; i<n; i++)
            if(!vis[i]&&lowcost[i]<Min)
            {
                Min=lowcost[i];
                k=i;
            }
        if(k==-1)break;
        vis[k]=true;
        for(int i=0; i<n; i++)
            if(!vis[i]&&lowcost[k]+mat[k][i]<lowcost[i])
            {
                lowcost[i]=lowcost[k]+mat[k][i];
                pre[i]=k;
            }
    }
}
int main()
{
    int n,t;
    // freopen("in.txt","r",stdin);
    scanf("%d",&t);
    while(t--)
    {
        memset(mat,0,sizeof(mat));
        memset(cost,0,sizeof(cost));
        scanf("%d%d",&n,&maxlong);
        for(int i=0; i<n+2; i++)
            scanf("%d%d",&point[i].x,&point[i].y);
        int a=point[1].x,b=point[1].y;
        int c=point[0].x,d=point[0].y;
        sort(point,point+n+2,cmp);
        for(int i=0; i<n+2; i++) //找到起点和终点
        {
            if(a==point[i].x&&b==point[i].y)
                end=i;
            if(c==point[i].x&&d==point[i].y)
                star=i;
        }
        for(int i=0; i<n+2; i++)//计算距离
            for(int j=0 ; j<n+2; j++)
                cost[i][j]=mathh(point[i].x,point[i].y,point[j].x,point[j].y);
        for(int i=0; i<n+2; i++)
        {
            map<double ,int > m;
            m.clear();
            for(int j=i+1; j<n+2; j++)//i点开始与j点的斜率
            {
                if(cost[i][j]<=maxlong)//先判断距离
                {
                    double xie=xielv(point[i].x,point[i].y,point[j].x,point[j].y);
                    if(m.find(xie)==m.end())//斜率先前没出现过
                    {
                        mat[i][j]=mat[j][i]=1;
                        m[xie]=1;
                    }
                    else
                        mat[i][j]=mat[j][i]=INF;
                }
                else
                    mat[i][j]=mat[j][i]=INF;
            }
        }
        Dijkstra(n+2,star);//最短路
        if(lowcost[end]==INF)//没有走到终点
            printf("impossible\n");
        else printf("%d\n",lowcost[end]-1);//走到终点的那一步不是加油站要-1；
    }
    return 0;
}