Point intersection(Line l1,Line l2){//面积比求出交点之比
Point ret = l1.s;
double t = ((l1.s.x - l2.s.x) * (l2.s.y - l2.e.y) - (l1.s.y - l2.s.y) * (l2.s.x - l2.e.x))
/((l1.s.x - l1.e.x) * (l2.s.y - l2.e.y) - (l1.s.y - l1.e.y) * (l2.s.x - l2.e.x));
ret.x += (l1.e.x - l1.s.x) * t;
ret.y += (l1.e.y - l1.s.y) * t;
return ret;
}