//正态分布函数，从三种生成法中随机选择一种，更加随机
public class  Normal {
  public Normal() {
  }

 public double normalRandom1(double a, double b) {//注意这里的b是方差，等于标准差的平方
 double temp = 12;
  double x = 0;
  for (int i = 0; i < temp; i++)
    x = x + (Math.random());
    x = (x - temp / 2) / (Math.sqrt(temp / 12));
   x = a + x * Math.sqrt(b); return x;
 }

 public double normalRandom2(double a, double b) {
    double pi = 3.1415926535;
    double r1 = Math.random();
    Math.random();
    Math.random();
    Math.random();
    Math.random();
    Math.random();
    Math.random();
    Math.random();
    double r2 = Math.random();
    double u = Math.sqrt((-2) * Math.log(r1)) * Math.cos(2 * pi * r2);
    double z = a + u * Math.sqrt(b); return (z);
  }

 public double normalRandom3(double a, double b) {
    double f = 0;
    double c0 = 2.515517, c1 = 0.802853, c2 = 0.010328;
    double d1 = 1.432788, d2 = 0.189269, d3 = 0.001308; double w;
    double r = Math.random();
    if (r <= 0.5) w = r;
      else w = 1 - r;
    if ((r - 0.5) > 0) f = 1;
    else if ((r - 0.5) < 0) f = -1;
   double y = Math.sqrt((-2) * Math.log(w));
    double x = f * (y - (c0 + c1 * y + c2 * y * y) / (1 + d1 * y + d2 * y * y + d3 * y * y * y));
    double z = a + x * Math.sqrt(b); return (z);
  }

  //然后判断用哪个正态分布随机数函数的方法如下：
  public double normalRandom(double a, double b) {
    double r = Math.random() * 9;
    switch ((int) r / 3) {
     case 0: return normalRandom1(a, b);
     case 1: return normalRandom2(a, b);
     case 2: return normalRandom3(a, b);
   }
     return 0.0;
   }
}