1、无穷级数计算 p = 1 - 1/3 + 1/5 -1/7+.....     π=4p
2、使用 Nilakantha 级数 π = 3 + 4/(2*3*4) - 4/(4*5*6) + 4/(6*7*8) - 4/(8*9*10) + 4/(10*11*12) - (4/(12*13*14) .....
3、假设有一个圆半径为1,所以四分之一圆面积就为PI,而包括此四分之一圆的正方形面积就为1,
如果随意的在正方形中投射飞标(点)好了,则这些飞标(点)有些会落于四分之一圆内,假设所投射的飞标(点)有n点,在圆内的飞标(点)有c点,则依比例来算。
public class Main {

    public static void main(String[] args) {
        gailv();
    }

    //无穷级数计算 1 - 1/3 + 1/5 -1/7+.....
    public static void wuqiongjishu() {
        double p1 = 1, p2 = 0;
        double i = 1;
        double flag = 1;
        double diff = Math.pow(0.1, 9);
        while (Math.abs(p1 - p2) >= diff) {
            i += 2;
            flag = -flag;
            p2 = p1;
            p1 = p1 + flag / i;
        }
        System.out.println(p1 * 4);
    }

    public static void gailv() {
        int N = (int)Math.pow(2,25);
        double a[] = new double[N];
        double b[] = new double[N];
        for (int i = 0; i < N; i++) {
            a[i] = Math.random();
            b[i] = Math.random();
        }
        int  cnt = 0;
        for (int i=0;i<N;i++){
            double c = a[i] * a[i] + b[i]*b[i];
            if (c<=1){
                cnt ++;
            }
        }
        System.out.println(4.0*cnt/N);
    }

}