478. Generate Random Point in a Circle -------M

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:

1. input and output values are in floating-point.
2. radius and x-y position of the center of the circle is passed into the class constructor.
3. a point on the circumference of the circle is considered to be in the circle.
4. 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.

A:

主要就是产生一个随机数：

C++ 中用 srand()  和 rand()

class Solution {
public:
    Solution(double radius, double x_center, double y_center) {
        r = radius;
        x = x_center,
        y = y_center;
        srand(static_cast<unsigned int>(std::time(nullptr)));
    }
    
    vector<double> randPoint() {
        double tmpX = rand();
        tmpX = (tmpX / RAND_MAX* 2 -1) *r; // in range [-1, 1]
        
        double tmpY = rand();
        tmpY = (tmpY / RAND_MAX* 2 -1) *r; // in range [-1, 1]
        
        if(tmpX * tmpX + tmpY * tmpY <= r*r){
            return vector<double>{x+tmpX,y+tmpY};
        }else{
            return randPoint();
        }
        
    }
private:
    double x, y;
    double r;
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(radius, x_center, y_center);
 * vector<double> param_1 = obj->randPoint();
 */