用三種方法來實現n!實現使用BigDecimal類,因爲用int最多正確算到12!,用long最多正確算到20!.實現方法都比較簡單,但是轉換爲BigDecimal實現後可能看起來有點莫名其妙。廢話不多說,直接上代碼:
第一種方法:從1開始,一直遞增乘到n,從而實現n!
- <span>package Factorial;
- import java.math.BigDecimal;
- import java.util.Scanner;
- public class Factorial1 {
- public static BigDecimal factorial(int n){
- BigDecimal result = new BigDecimal(1);
- BigDecimal a;
- for(int i = 2; i <= n; i++){
- a = new BigDecimal(i);//將i轉換爲BigDecimal類型
- result = result.multiply(a);//不用result*a,因爲BigDecimal類型沒有定義*操作</span><span>
- }
- return result;
- }
- public static void main(String[] arguments){
- Scanner sc = new Scanner(System.in);
- int a = sc.nextInt();//讀取控制檯輸入的整數
- System.out.println(a + "!=" + factorial(a));
- }
- }
- </span>
第二種方法:從n開始,一直遞減乘到2,從而實現n!
- <span>package Factorial;
- import java.math.BigDecimal;
- import java.util.Scanner;
- public class Factorial3 {
- public static BigDecimal factorial(BigDecimal n){
- BigDecimal bd1 = new BigDecimal(1);//BigDecimal類型的1
- BigDecimal bd2 = new BigDecimal(2);//BigDecimal類型的2</span><span>
- BigDecimal result = bd1;//結果集,初值取1
- while(n.compareTo(bd1) > 0){//參數大於1,進入循環
- result = result.multiply(n.multiply(n.subtract(bd1)));//實現result*(n*(n-1))
- n = n.subtract(bd2);//n-2後繼續
- }
- return result;
- }
- public static void main(String[] arguments){
- Scanner sc = new Scanner(System.in);
- BigDecimal n = sc.nextBigDecimal();
- System.out.print(n + "!=" + factorial(n));
- }
- }
- </span>
第三種方法,用遞歸實現:f(n)=n*f(n-1),f(1)=1,遞歸相乘,從而實現n!
- package Factorial;
- import java.util.Scanner;
- import java.math.BigDecimal;
- public class Factorial2 {
- public static BigDecimal factorial(BigDecimal n){
- BigDecimal bd1 = new BigDecimal(1);//1
- if(n.equals(new BigDecimal(1))){
- return bd1;
- }
- else
- return n.multiply(factorial(n.subtract(bd1)));//n*f(n-1)
- }
- public static void main(String[] arguments){
- Scanner sc = new Scanner(System.in);
- BigDecimal a = sc.nextBigDecimal();
- BigDecimal result = factorial(a);
- System.out.println(a + "!=" +result);
- }
- }