Pascal's 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 : First rows of Pascal's triangle.
 * Input   : One integer n (rows).
 * Output  : n lines of numbers.
 * Approach: Build each row from the previous row.
 * Time    : O(n^2)
 * Space   : O(n)
 * 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: Keep the previous row. The first row has no previous row, so we start with an empty array.
        int[] previous = new int[0];

        // Step 2: Repeat once for each row. Row numbers go from 1 to rows.
        for (int row = 1; row <= rows; row++) {
            // Step 3: Row i has i numbers. The first and last number are always 1.
            int[] current = new int[row];
            current[0] = 1;
            current[row - 1] = 1;

            // Step 4: Each middle number is the sum of the two numbers above it.
            for (int position = 1; position < row - 1; position++) {
                current[position] = previous[position - 1] + previous[position];
            }

            // Step 5: Print the row with one space between numbers.
            StringBuilder line = new StringBuilder();
            for (int position = 0; position < row; position++) {
                if (position > 0) {
                    line.append(" ");
                }
                line.append(current[position]);
            }
            System.out.println(line);

            // Step 6: The current row becomes the previous row for the next loop.
            previous = current;
        }
    }
}