学步园

线段相交 参考算法导论P597

struct point{  
    double x,y;  
};  
  
struct bian{      
    point a,b;  
};  
  
double xmult(point a,point b,point c)//大于零代表a,b,c左转  
{    
    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);    
}    
  
bool OnSegment(point a,point b,point c)//a,b,c共线时有效   
{            
    return c.x>=min(a.x,b.x)&&c.x<=max(a.x,b.x)&&c.y>=min(a.y,b.y)&&c.y<=max(a.y,b.y);      
}    
    
bool cross(point a,point b,point c,point d){//判断ab 与cd是否相交     
    double d1,d2,d3,d4;    
    d1=xmult(c,d,a);    
    d2=xmult(c,d,b);    
    d3=xmult(a,b,c);    
    d4=xmult(a,b,d);    
    if(d1*d2<0&&d3*d4<0)  return 1;    
    else    if(d1==0&&OnSegment(c,d,a)) return 1;    
    else    if(d2==0&&OnSegment(c,d,b)) return 1;    
    else    if(d3==0&&OnSegment(a,b,c)) return 1;    
    else    if(d4==0&&OnSegment(a,b,d)) return 1;    
    return 0;    
}     

hdoj 1086