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;
}