Given the radius and x-y positions of the center of a circle, write a function
randPoint
which generates a uniform random point in the circle.Note:
- input and output values are in floating-point.
- radius and x-y position of the center of the circle is passed into the class constructor.
- a point on the circumference of the circle is considered to be in the circle.
randPoint
returns a size 2 array containing x-position and y-position of the random point, in that order.
Example 1:
Input: ["Solution","randPoint","randPoint","randPoint"] [[1,0,0],[],[],[]] Output: [null,[-0.72939,-0.65505],[-0.78502,-0.28626],[-0.83119,-0.19803]]
Example 2:
Input: ["Solution","randPoint","randPoint","randPoint"] [[10,5,-7.5],[],[],[]] Output: [null,[11.52438,-8.33273],[2.46992,-16.21705],[11.13430,-12.42337]]
Explanation of Input Syntax:
The input is two lists: the subroutines called and their arguments.
Solution
's constructor has three arguments, the radius, x-position of the center, and y-position of the center of the circle.randPoint
has no arguments. Arguments are always wrapped with a list, even if there aren't any.
Approach #1: Math. [Java]
class Solution { double radius, x_center, y_center; public Solution(double radius, double x_center, double y_center) { this.radius = radius; this.x_center = x_center; this.y_center = y_center; } public double[] randPoint() { double r = Math.sqrt(Math.random()) * radius; double deg = Math.random() * 2 * Math.PI; double x = x_center + r * Math.cos(deg); double y = y_center + r * Math.sin(deg); return new double[]{x, y}; } } /** * Your Solution object will be instantiated and called as such: * Solution obj = new Solution(radius, x_center, y_center); * double[] param_1 = obj.randPoint(); */
Analysis:
The task is to generate uniformly distributed numbers within a circle of radius P in the (x, y) plane. At first polar coordinates seems like a great idea, and the naive solution is to pick a radius r uniformly distributed in [0, R], and then an angle theta uniformly distributed in [0, 2*PI]. But, you end up with an exess of points near the origin (0, 0)! This is wrong because if we look at a certain angle interval, say [theta, theta+dtheda], there needs to be more points generated further out (at large r), than close to zero. The radius must not be picked from a uniform distribution, but one that foes as pdf_r = (2 / R^2) * r
That's easy enough to do by calculating the inverse of the cumulative distribution, and we get for r:
r = R * sqrt(rand())
where rand() is a uniform random number in [0, 1].
Analysis:
https://leetcode.com/problems/generate-random-point-in-a-circle/