POJ 2653 Pick-up sticks 判断线段相交

枚举每条线段 这条线段上面没有与它相交的线段

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#define eps 1e-8
#define INF 1e9
using namespace std;

const int maxn=200000+10;
int ans[1005];
int n,cnt;

typedef struct Point
{
    double x,y;
    Point() {};
    Point(double xx,double yy)
    {
        x=xx;
        y=yy;
    }
} Vector;

int sgn(double x)
{
    if(fabs(x) < eps)return 0;
    if(x < 0)return -1;
    else return 1;
}

Point pot[maxn];

double crs_prdct(Vector a,Vector b)
{
    return a.x*b.y-b.x*a.y;
}

Vector operator - (Point a,Point b)
{
    return Vector(a.x-b.x,a.y-b.y);
}

int intersect(Point p1,Point p2,Point q1,Point q2)
{
    return
        max(p1.x,p2.x) >= min(q1.x,q2.x) &&
        max(q1.x,q2.x) >= min(p1.x,p2.x) &&
        max(p1.y,p2.y) >= min(q1.y,q2.y) &&
        max(q1.y,q2.y) >= min(p1.y,p2.y) &&
        sgn(crs_prdct(q1-p1,p2-p1))*sgn(crs_prdct(q2-p1,p2-p1)) <= 0 &&
        sgn(crs_prdct(p1-q1,q2-q1))*sgn(crs_prdct(p2-q1,q2-q1)) <= 0;
}

int main()
{
//    freopen("in.txt","r",stdin);
    while(scanf("%d",&n),n)
    {
        double x1,y1,x2,y2;
        for(int i=0; i<n; i++)
        {
            scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
            pot[2*i]=Point(x1,y1);
            pot[2*i+1]=Point(x2,y2);
        }
        cnt=0;
        for(int i=0; i<n; i++)
        {
            bool flag=true;
            for(int j=i+1; j<n; j++)
            {
                if(intersect(pot[2*i],pot[2*i+1],pot[2*j],pot[2*j+1]))
                {
                    flag=false;
                    break;
                }
            }
            if(flag) ans[cnt++]=i+1;
            if(cnt>1000) break;
        }
        sort(ans,ans+cnt);
        printf("Top sticks:");
        for(int i=0; i<cnt-1; i++)
            printf(" %d,",ans[i]);
        printf(" %d.\n",ans[cnt-1]);
    }
    return 0;
}