我们做两次

每次把一条线段视为直线，判断另一条线段的两个点是否在直线的两侧

如果两次都符合，说明直线相交

struct Point {double x, y; Point operator - (const Point &A) const {Point B; B.x=x-A.x; B.y=y-A.y; return B; }double operator ^ (const Point &A) const {return x * A.y - y * A.x; }
};
namespace Cross {bool Line_cross(Point p1, Point p2, Point p3, Point p4) {double a1 = (p1 - p2) ^ (p3 - p2); double a2 = (p1 - p2) ^ (p4 - p2); if(a1 * a2 >= 0) return false; return true; }bool cross(Point p1, Point p2, Point p3, Point p4) {bool t1 = Line_cross(p1, p2, p3, p4); bool t2 = Line_cross(p3, p4, p1, p2); return t1 && t2; }
}