C语言解答

#include "stdio.h"  
#include "math.h"  
main()  
{long n,sum=0,i=1,s=2;  
scanf("%d",&n);  
while(i<=n)  
{sum=sum+s;s=s+2*pow(10,i);  
i++;}  
printf("%ld",sum);  
}

C++解答

#include<iostream>
#include<math.h>
using namespace std;
int main()
{
	int s[100],b[100],i,a,j=0,d=0;
	cin>>a;
	b[0]=0;
	for(i=0;i<a;i++)
	{
		b[j]=2*pow(10,(float)i);
		j++;
	}
	for(i=0;i<a;i++)
	{
			if(i==0)
			{
				s[i]=b[i];
				continue;
			}
			s[i]=b[i]+s[i-1];
	}
	for(i=0;i<a;i++)
	{
		d+=s[i];
	}
	cout<<d<<endl;
}

Java解答

import java.util.Scanner;

public class Main {
  private static Scanner s = new Scanner(System.in) ;
  
  public static void main(String[] args) {
	 int n = s.nextInt() ;
	 int sum = 0 ;
	 int temp = 0 ;
	 for (int i = 1; i <= n; i++) {
		int t = 1 ;
		for (int j = 1; j <= i; j++) {
			
			temp += 2*t ;
			t*=10 ;
		}
		sum+=temp ;
		temp = 0 ;
	  }
	 
	 System.out.println(sum) ;
  }
}

Python解答

a = input()
b = 0
for i in range(1, a + 1):
    c = 2
    for j in range(1, i):
        c *= 10
        c += 2
    b+=c
print b