Reverse Alphabet Triangle

Java 8 has no String.repeat, so we use a small helper method. In Java 11 and newer you can write " ".repeat(count).

/*
 * Problem : Triangle of letters going backward.
 * Input   : One integer n (rows).
 * Output  : n lines of letters.
 * Approach: Start from the n-th letter and count down.
 * Time    : O(n^2)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int rows = new Scanner(System.in).nextInt();

        // Step 1: Find the last letter that we need. Example: n = 4 -> D.
        char lastLetter = (char) ('A' + rows - 1);

        // Step 2: Repeat once for each row. Row numbers go from 1 to rows.
        for (int row = 1; row <= rows; row++) {
            StringBuilder line = new StringBuilder();

            // Step 3: Write "row" letters. Start from the last letter and go backward.
            for (int step = 0; step < row; step++) {
                line.append((char) (lastLetter - step));
            }

            // Step 4: Print the finished line.
            System.out.println(line);
        }
    }
}