模板：判断两线段是否相交

 1 inline double CrossProduct(node a, node b, node c){
 2     return (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
 3 }
 4 //Calculate the crossproduct
 5 
 6 
 7 inline bool SegX(node p1, node p2, node p3, node p4){
 8     double d1 = CrossProduct(p3, p4, p1);
 9     double d2 = CrossProduct(p3, p4, p2);
10     double d3 = CrossProduct(p1, p2, p3);
11     double d4 = CrossProduct(p1, p2, p4);
12     return (d1 * d2 <= 0 && d3 * d4 <= 0);
13 }
14 //Judge whether the line segments intersact