C++ :

#include <iostream>
using namespace std;
int main() {
 int n;
 cin >> n;
 int ch = 0;
 for (int i = 1; i <= n; i++) {
 for (int j = 1; j <= i; j++)
 cout << (char)('A' + (ch++) % 26);
 cout << endl;
 }
 return 0;
}