题解 | 直线与圆交点间距

这一题的恶心之处在于直线L的A，B两点只是直线上普通的两点，我一开始把它错以为是与圆的交点。

我们应该通过前面某一题的方法求出点到直线的距离，然后再通过勾股定理，求出AB的距离，记得要乘二

import java.util.Scanner;

class Point {
    double x, y;
    Point(double A, double B) {
        x = A;
        y = B;
    }
}

class Line {
    Point point_A, point_B;
    Line(Point A, Point B) {
        point_A = A;
        point_B = B;
    }
}

class Circle {
    Point O;
    int r;
    Circle(Point A, int B) {
        O = A;
        r = B;
    }
}

public class Main {
    static double distance(Point a,Point b) {
		double d=Math.sqrt(Math.pow(a.x-b.x, 2)+Math.pow(a.y-b.y, 2));
		return d;
	}
    public static double getDistance(Circle circle, Line l) {
        // 请在这里实现你的代码
        double ao=distance(l.point_A, circle.O);
		double ab=distance(l.point_A, l.point_B);
		double ob=distance(circle.O, l.point_B);
		double cosA=(ao*ao+ab*ab-ob*ob)/(2*ao*ab);
		double sinA=Math.sqrt(1-cosA*cosA);
		double d=ao*sinA;
		if(d>=circle.r) {
			return 0;
		}
			return 2*Math.sqrt(circle.r*circle.r-d*d);
		

    }
    

















































































































































































    
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        double ox = scanner.nextDouble();
        double oy = scanner.nextDouble();
        int r = scanner.nextInt();
        
        double x1 = scanner.nextDouble();
        double y1 = scanner.nextDouble();
        double x2 = scanner.nextDouble();
        double y2 = scanner.nextDouble();
        
        Point center = new Point(ox, oy);
        Circle circle = new Circle(center, r);
        
        Point p1 = new Point(x1, y1);
        Point p2 = new Point(x2, y2);
        Line l = new Line(p1, p2);
        
        double result = getDistance(circle, l);
        System.out.printf("%.6f\n", result);
        
        scanner.close();
    }
}