ACM模版

判断线段相交

const double eps = 1e-10;

struct point
{
    double x, y;
};

double min(double a, double b)
{
    return a < b ? a : b;
}

double max(double a, double b)
{
    return a > b ? a : b;
}

bool inter(point a, point b, point c, point d)
{
    if (min(a.x, b.x) > max(c.x, d.x) || min(a.y, b.y) > max(c.y, d.y) || min(c.x, d.x) > max(a.x, b.x) || min(c.y, d.y) > max(a.y, b.y))
    {
        return 0;
    }
    double h, i, j, k;
    h = (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
    i = (b.x - a.x) * (d.y - a.y) - (b.y - a.y) * (d.x - a.x);
    j = (d.x - c.x) * (a.y - c.y) - (d.y - c.y) * (a.x - c.x);
    k = (d.x - c.x) * (b.y - c.y) - (d.y - c.y) * (b.x - c.x);
    return h * i <= eps && j * k <= eps;
}