CodingGyan

Alphabet Triangle Pattern

beginner Pattern Printing

Problem

Read n (1 to 26) and print a triangle of letters. Row i has the first i letters of the alphabet: A, AB, ABC, and so on.

Constraints

  • 1 <= n <= 26

Examples

Input

3

Output

A
AB
ABC

Solutions

Best approachTime O(n^2)Space O(1)

Letters are stored as numbers. A is the first letter, so A plus an index gives the next letters in the alphabet. In Java, a char is a number inside the computer. 'A' + 1 is the number of 'B', so we cast the result back to char.

Same code works in Java810172125Basic features only, so nothing changes between versions.

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.
 * Input   : One integer n (rows).
 * Output  : n lines of letters.
 * Approach: Convert a number to a letter.
 * 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++) {
            StringBuilder line = new StringBuilder();

            // Step 2: Write the first "row" letters. Letter 0 is A, letter 1 is B, and so on.
            for (int index = 0; index < row; index++) {
                // Step 3: 'A' + index is a number. We cast it back to a char to get the letter.
                line.append((char) ('A' + index));
            }

            // Step 4: Print the finished line.
            System.out.println(line);
        }
    }
}
Hints (2)
  1. Use an outer loop for rows and an inner loop for the letters or digits.
  2. Write the output for n = 3 and look for the rule.

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