给定圆的半径和圆心的位置，实现函数 randPoint ，在圆中产生均匀随机点。

    实现 Solution 类:

    Solution(double radius, double x_center, double y_center) 用圆的半径 radius 和圆心的位置 (x_center, y_center) 初始化对象
    randPoint() 返回圆内的一个随机点。圆周上的一点被认为在圆内。答案作为数组返回 [x, y] 。

    示例 1：

    输入:
    [“Solution”,”randPoint”,”randPoint”,”randPoint”]
    [[1.0, 0.0, 0.0], [], [], []]
    输出: [null, [-0.02493, -0.38077], [0.82314, 0.38945], [0.36572, 0.17248]]
    解释:
    Solution solution = new Solution(1.0, 0.0, 0.0);
    solution.randPoint ();//返回[-0.02493，-0.38077]
    solution.randPoint ();//返回[0.82314,0.38945]
    solution.randPoint ();//返回[0.36572,0.17248]

    提示：

    0 < radius <= 108
    -107 <= x_center, y_center <= 107
    randPoint 最多被调用 3 * 104 次

    1. class Solution {
    2.     double r, x, y;
    3.     Random random = new Random();
    4.     public Solution(double radius, double x_center, double y_center) {  
    5.         r = radius; x = x_center; y = y_center;
    6.     }
    8.     public double[] randPoint() {
    9.         double len = Math.sqrt(random.nextDouble(r * r)), ang = random.nextDouble(2 * Math.PI);
    10.         double nx = x + len * Math.cos(ang), ny = y + len * Math.sin(ang);
    11.         return new double[]{nx, ny};
    12.     }
    13. }
    15. /**
    16.  * Your Solution object will be instantiated and called as such:
    17.  * Solution obj = new Solution(radius, x_center, y_center);
    18.  * double[] param_1 = obj.randPoint();
    19.  */