Reverse Number 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 : Upside-down number triangle.
 * Input   : One integer n (rows).
 * Output  : n lines of numbers.
 * Approach: Each row is one number shorter than the row before.
 * 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: Repeat once for each row. Row numbers go from 1 to rows.
        for (int row = 1; row <= rows; row++) {
            // Step 2: Count how many numbers this row has. Row 1 has n numbers, the last row has 1.
            int count = rows - row + 1;
            StringBuilder line = new StringBuilder();

            // Step 3: Write the numbers 1, 2, ... up to the count.
            for (int number = 1; number <= count; number++) {
                if (number > 1) {
                    line.append(" ");
                }
                line.append(number);
            }

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