向量叉积判断两线段是否相交

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <iostream>

typedef struct node
{
    double x,y;
}point;

point p1,p2,q1,q2;
double result1,result2;

double crossProduct(point a,point b1,point b2)
{
    double x1,y1,x2,y2;
    x1=a.x-b1.x;
    y1=a.y-b1.y;
    x2=b2.x-b1.x;
    y2=b2.y-b1.y;
    return x1*y2-x2*y1;
}

int main()
{
    while(1)
    {
        scanf("%lf%lf",&p1.x,&p1.y);
        scanf("%lf%lf",&p2.x,&p2.y);
        scanf("%lf%lf",&q1.x,&q1.y);
        scanf("%lf%lf",&q2.x,&q2.y);

        result1=crossProduct(p1,q1,q2);//注意向量顺序
        result2=crossProduct(p2,q2,q1);
        printf("%lf  %lf\n",result1,result2);

        if(result1*result2>=0)printf("YES\n");//如果叉乘大于0则相交
        else printf("NO\n");
    }
    return 0;
}