C语言程序设计教程（第三版）课后习题6.3 （Java代码）

//普通方法

import java.util.Scanner;

public class Main {

        public static void n(int num){

                int Sn = 0;

                int sum = 1;

                int M=0;

                for(int i = 0;i<num;i++){

                        if(i!=0){

                                sum = sum*10;

                                Sn = Sn+2*sum;

                                M+=Sn;

                        }else{

                                Sn = Sn+2;

                                M+=Sn;

                        }

              }

              System.out.print(M);

        }

        public static void main(String[] arg){

                Scanner sc = new Scanner(System.in);

                int num = sc.nextInt();

                n(num);

        }

}

//API方法

import java.util.Scanner;

public class Main {

        public static void n(int num){

                int Sn = 0;

                int M=0;

                for(int i=0;i<num;i++ ){

                        Sn = (int) (Sn +2*Math.pow(10, i));

                        M+=Sn;

                }

                System.out.print(M);

        }

        public static void main(String[] arg){

                Scanner sc = new Scanner(System.in);

                int num = sc.nextInt();

                n(num);

        }

}