C++ :

#include <bits/stdc++.h>
using namespace std;


int main(){
	int n;
	cin>>n;
	for(int i = 1;i <= n;i++){
		for(int j = 1;j <= n;j++){
			cout<<(char)(j+64);
		}
		cout<<endl;
	}
}

Java :

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
//        String c = scanner.next();
        int num1 = scanner.nextInt();
        for (char i = 0; i < num1; i++) {
            for(char j = 'A';j < num1 + 'A';j++)
            System.out.print(j);
            System.out.println("");
        }




    }public static void paiXu(int[] a,int length){
        int temp;
        for(int i = 0;i < length - 1;i++){
            for(int k = 0;k < length - i - 1;k++){
                if(a[k] > a[k + 1]){
                    temp = a[k];
                    a[k] = a[k + 1];
                    a[k + 1] = temp;
                }
            }
        }
    }
    public static int er(int n)
    {
        if(n==0)
        {return 0;}
        int f1=1,f2;
        n--;
        while(n != 0)
        {
            f2=f1*2;
            f1=f2;
            n--;
        }
        return f1;
    }

    public static void paiXud(int[] a){
        int temp;
        for(int i = 0;i < a.length - 1;i++){
            for(int k = 0;k < a.length - i - 1;k++){
                if(a[k] < a[k + 1]){
                    temp = a[k];
                    a[k] = a[k + 1];
                    a[k + 1] = temp;
                }
            }
        }
    } public static void paiXux(int[] a){
        int temp;
        for(int i = 0;i < a.length - 1;i++){
            for(int k = 0;k < a.length - i - 1;k++){
                if(a[k] > a[k + 1]){
                    temp = a[k];
                    a[k] = a[k + 1];
                    a[k + 1] = temp;
                }
            }
        }
    }

    public static int MAX2(int[][] a){
        int max = a[0][0];
        for (int i = 0; i < a.length; i++) {
            for(int j = 0;j < a[0].length;j++){
                if(max < a[i][j])max = a[i][j];
            }
        }
        return max;
    }public static int MIN2(int[][] a){
        int min = a[0][0];
        for (int i = 0; i < a.length; i++) {
            for(int j = 0;j < a[0].length;j++){
                if(min > a[i][j])min = a[i][j];
            }
        }
        return min;
    }
    public static int MAX(int[] a){
        int max = a[0];
        for (int i = 0; i < a.length; i++) {
            if(max < a[i]){
                max = a[i];
            }
        }
        return max;
    }

    public static int MIN(int[] a){
        int min = a[0];
        for (int i = 0; i < a.length; i++) {
            if(min > a[i]){
               min = a[i];
            }
        }
        return min;
    }
    public static int sum(int a){
        int x,s = 0;
        while(a != 0){
            x = a % 10;
            s = s + x;
            a /= 10;
        }
        return s;
    }
    public static boolean isPrime(int a){
        if(a <= 1)return false;
        else
        for(int i = 2;i <= Math.sqrt(a);i++){
            if(a % i == 0)return false;
        }return true;
    }

    public static long qiuYu(long i){
        long a;
        a = i % 10;
        return a;
    }
    public static int daoXu(int x){
        int s = 0,m;
        while(x != 0){
            m = x % 10;
            s = s * 10 + m;
            x /= 10;
        }
        return s;
    }

    public static int jiaWei(int x){
        int s = 0,m;
        while(x != 0){
            m = x % 10;
            s = s + m;
            x /= 10;
        }
        return s;
    }

    public static int fun(int x){
        if(x == 1 || x == 2)return 1;
        else return fun(x - 1) + fun(x - 2);
    }
}

Python :

n=int(input())

for i in range(n):
  c='A'
  for j in range(n):
    print(c,end="")
    c=chr(ord(c)+1)
  print()