线段交点模版

http://acm.hdu.edu.cn/showproblem.php?pid=1086

跨立实验算法

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
using namespace std;
struct Point
{
    double x,y;
} ;
struct Line
{
    Point s,e;
};
Line arr[105];
double Chacheng(Point a,Point b,Point c)
{
    return (c.x-a.x)*(b.y-a.y)-(b.x-a.x)*(c.y-a.y);
}
int is_across(Line a,Line b)
{
    if(max(a.s.x,a.e.x)>=min(b.s.x,b.e.x)&&
       max(b.s.x,b.e.x)>=min(a.s.x,a.e.x)&&
       max(a.s.y,a.e.y)>=min(b.s.y,b.e.y)&&
       Chacheng(b.s,a.e,a.s)*Chacheng(a.e,b.e,a.s)>=0&&
       Chacheng(a.s,b.e,b.s)*Chacheng(b.e,a.e,b.s)>=0)
    return 1;
    return 0;
}
int main()
{
    //freopen("in.txt","r",stdin);
    int n;
    while(scanf("%d",&n)&&n)
    {
        int ans=0;
        for(int i=0;i<n;i++)
            scanf("%lf %lf %lf %lf",&arr[i].s.x,&arr[i].s.y,
                                    &arr[i].e.x,&arr[i].e.y);
        for(int i=0;i<n-1;i++)
            for(int j=i+1;j<n;j++)
                if(is_across(arr[i],arr[j]))
                    ans++;
        printf("%d\n",ans);
    }
    return 0;
}