解析几何，注意除数为零的时候特殊处理

解析几何，注意除数为零的时候特殊处理，即平行于x轴或y轴：

import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        for(int i = 0; i < n; ++i){
            double a = sc.nextDouble(), b = sc.nextDouble(), c = sc.nextDouble(), d = sc.nextDouble();
            double t1 = a, t2 = b;    //记录(a, b)的初值
            c = c - a;    //把(a, b)平移到(0, 0)，当然(c, d)也跟着平移
            d = d - b;
            a = 0;
            b = 0;
            if(d == 0){    //除数为零的时候特殊处理
                double x1 = c/2, x2 = c/2, y1 = x1*Math.sqrt(3), y2 = -y1;
                x1 += t1;    //最后把答案都平移回来
                x2 += t1;
                y1 += t2;
                y2 += t2;
                f(x1, y1, x2, y2);
                continue;
            }
            double k = c*c/(2*d) + d/2.0;
            double A = 1 + c*c/(d*d), B = -2*k*c/d, C = k*k-c*c-d*d;
            double x1 = (-B + Math.sqrt(B*B-4*A*C))/(2*A);    //求根公式
            double x2 = (-B - Math.sqrt(B*B-4*A*C))/(2*A);
            double y1 = k - c/d*x1, y2 = k - c/d*x2;
            x1 += t1;    ////最后把答案都平移回来
            x2 += t1;
            y1 += t2;
            y2 += t2;
            f(x1, y1, x2, y2);
        }
    }
    static void f(double x1, double y1, double x2, double y2){    //先打印较小坐标
        if(x1 < x2) System.out.printf("%.2f %.2f %.2f %.2f\n", x1, y1, x2, y2);
        if(x1 > x2) System.out.printf("%.2f %.2f %.2f %.2f\n", x2, y2, x1, y1);
        if(x1 == x2){
            if(y1 < y2) System.out.printf("%.2f %.2f %.2f %.2f\n", x1, y1, x2, y2);
            else System.out.printf("%.2f %.2f %.2f %.2f\n", x2, y2, x1, y1);
        }
    }
}