478. Generate Random Point in a Circle 

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class Solution {

    private final double radius;
    private final double x_center;
    private final double 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() {
        while (true) {
            double randomX = ThreadLocalRandom.current().nextDouble(1.0000001) * 2 * radius - radius;
            double randomY = ThreadLocalRandom.current().nextDouble(1.0000001) * 2 * radius - radius;
            if (randomX * randomX + randomY * randomY <= radius * radius) {
                return new double[]{x_center + randomX, y_center + randomY}; // 不要忘记根据圆心偏移
            }
        }
    }

}

References

478. Generate Random Point in a Circle