階乘：

例如：

2的階乘：1x2

4的階乘：1x2x3x4

package com.wochat.controller;

public class MainClass {
    public static void main(String[] args) {


        long sum = 1;
        int score = 10;

        for (int i = 1; i <= score; i++) {
            sum *= i;
        }

        System.out.println(sum);
    }
}
 

 10的階乘：3628800

如果，求100的階乘：93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000
 

其值超出long的範圍

        import java.math.BigInteger;

        int s=100;
        BigInteger a=new BigInteger("1");
        for (int i = 1; i <=s; i++) {
            BigInteger b=new BigInteger(String.valueOf(i));
            a=a.multiply(b);
        }
        System.out.println(a);
 

String s="93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000";
        System.out.println(s.length());

100的階乘是158位

當然，在階乘這種問題下，遞迴是最裝B的

無形的裝叉最致命！

package com.wochat.controller;

public class MainClass {
    public static void main(String[] args) {

        System.out.println(vsum(10));
    }

    public static int vsum(int v){
        int sum;
        if (v<=1){
            sum=v;
        }
        else{
            sum=v*vsum(v-1);
        }
        return sum;
    }

}
 

改動一下： 

package com.wochat.controller;


public class MainClass {

    public static void main(String[] args) {

        System.out.println(vsum(-1));
    }

    public static int vsum(int v) {
        if (v <= 1) {
            return v;
        } else {
            return v * vsum(v - 1);
        }
    }


}

最為簡單的九九乘法表

        for (int i = 1; i <= 9; i++) {
            for (int j = 1; j <=9; j++) {
                System.out.print(i+"x"+j+"="+i*j+"\t");
            }
            System.out.println();
        }

1x1=1	
2x1=2	2x2=4	
3x1=3	3x2=6	3x3=9	
4x1=4	4x2=8	4x3=12	4x4=16	
5x1=5	5x2=10	5x3=15	5x4=20	5x5=25	
6x1=6	6x2=12	6x3=18	6x4=24	6x5=30	6x6=36	
7x1=7	7x2=14	7x3=21	7x4=28	7x5=35	7x6=42	7x7=49	
8x1=8	8x2=16	8x3=24	8x4=32	8x5=40	8x6=48	8x7=56	8x8=64	
9x1=9	9x2=18	9x3=27	9x4=36	9x5=45	9x6=54	9x7=63	9x8=72	9x9=81	

        for (int i = 1; i <= 9; i++) {
            for (int j = 1; j <=i; j++) {
                System.out.print(i+"x"+j+"="+i*j+"\t");
            }
            System.out.println();
        }

 對比一下兩段程式碼，改動一下啊，就有不同的效果

這就是Java的神奇之處！