C++ :

#include <iostream>
#include <fstream>
#include <string>
#include <cmath>
using namespace std;
const int maxn = 200;
char seq[maxn] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
char print_seq[maxn][maxn];
int n;

void f(int idx, int len, int x, int y){

    if(idx == -1){
        return ;
    }       
    for(int dx = -(len); dx <= (len); dx += len ){
        for(int dy = -(len); dy <= (len); dy += len){
            int xx = x + dx;
            int yy = y + dy;

            print_seq[xx][yy] = seq[idx];

        }
    }

    f(idx - 1, len + 1, x, y);
}

int main(){
//  fstream cin("a.txt");
    cin>>n; 
    for(int i = 0; i < (2 * n - 1); ++i){
        for(int j = 0; j < (2 * n - 1); ++j){
            print_seq[i][j] = '.';
        }
    }

    f(n - 2, 1, n - 1, n - 1);

    print_seq[n - 1][n - 1] = seq[n - 1];

    for(int i = 0; i < (2 * n - 1); ++i){
        for(int j = 0; j < (2 * n - 1); ++j){
            cout<<print_seq[i][j];
        }
        cout<<endl;
    }   

    return 0;
}

Java :

import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		//接收用户输入的“边长”
		n = sc.nextInt();
		//如果边长为1，直接输出得了
		if(n == 1){
			System.out.println("A");
			return;
		}
		int b = n*2-1;
		char arr[][] = new char[b][b];
		for(int i = 0; i < b; i++){
			for(int j = 0; j < b; j++){
				arr[i][j] = '.';
			}
		}
		
		f(arr, b);
		
		//打印结果图像
		for(int i = 0; i < b; i++){
			for(int j = 0; j < b; j++){
				System.out.print(arr[i][j]);
			}
			System.out.println();
		}
	}
	
	static int n;
	static void f(char ch[][], int b){
		int start = 0;
		int end = b-1;
		char w= 'A';
		int add = b/3+n/3;
		
		int cnt = 0;
		while(true){
			//由外向内
			for(int i = start; i <= end; i+=add){
				ch[i][start] = w;//左
				ch[start][i] = w;//上
				ch[i][end] = w;//右
				ch[end][i] = w;//下
			}
			start++;
			end--;
			w++;
			add = add - 1;
			if(add <= 0)add =1;
			
			cnt++;
			if(cnt == n)break;
		}
	}
}