题解 | 两直线交点

import java.util.*;

public class Main {
    static class Point {
        public double x, y;
        public Point(double x, double y) {
            this.x = x;
            this.y = y;
        }
        public Point() {
            this(0.0, 0.0);
        }
    }

    static class Line {
        public Point point_A, point_B;
        public Line(Point a, Point b) {
            point_A = a;
            point_B = b;
        }
        public Line() {
            point_A = new Point();
            point_B = new Point();
        }
    }

    public static Point findMeetingPoint(Line lineA, Line lineB) {
        // TODO: 求直线 lineA 与 lineB 的交点
        double ax = lineA.point_A.x;
        double ay = lineA.point_A.y;
        double bx = lineA.point_B.x;
        double by = lineA.point_B.y;
        double cx = lineB.point_A.x;
        double cy = lineB.point_A.y;
        double dx = lineB.point_B.x;
        double dy = lineB.point_B.y;
        Point jiaodian = new Point();

        if(ax != bx && cx != dx){
            double k1 = (ay - by) / (ax - bx);
            double b1 = by - k1*bx;

            double k2 = (cy - dy) / (cx - dx);
            double b2 = dy - k2*dx;
            if(k1 == k2){
            jiaodian.x = -1;
            jiaodian.y = -1;
            return jiaodian;
            }
            jiaodian.x = (b2 - b1) / (k1 - k2);
            jiaodian.y = k1 * jiaodian.x + b1;
        }else if(ax != bx && cx == dx){
            double k1 = (ay - by) / (ax - bx);
            double b1 = by - k1*bx;
            jiaodian.x = cx;
            jiaodian.y = k1 * jiaodian.x + b1;
        }else if(ax == bx && cx != dx){
            double k1 = (cy - dy) / (cx - dx);
            double b1 = dy - k1*dx;
            jiaodian.x = ax;
            jiaodian.y = k1 * jiaodian.x + b1;
        }else{
            jiaodian.x = -1;
            jiaodian.y = -1;
        }
        return jiaodian;
    }





































































































































































    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        Point A = new Point();
        A.x = scanner.nextDouble();
        A.y = scanner.nextDouble();
        Point B = new Point();
        B.x = scanner.nextDouble();
        B.y = scanner.nextDouble();
        Point C = new Point();
        C.x = scanner.nextDouble();
        C.y = scanner.nextDouble();
        Point D = new Point();
        D.x = scanner.nextDouble();
        D.y = scanner.nextDouble();
        Line AB = new Line(A, B);
        Line CD = new Line(C, D);
        Point ans = findMeetingPoint(AB, CD);
        System.out.println(ans.x + " " + ans.y);
        scanner.close();
    }
}