Pattern-based coding questions are a common way to assess a candidate’s logical thinking, loop understanding, and ability to derive mathematical relationships from a problem statement.

One such question is the Reverse Pyramid Pattern.

Problem Statement

Write a Java program that takes an integer N as input and prints a reverse pyramid.

Example Input

5

Expected Output

* * * * *
 * * * *
  * * *
   * *
    *

Requirements

1. The first row should contain N stars separated by spaces.
2. Each subsequent row should:
   • Have one additional leading space.
   • Have one less star than the previous row.
3. The output should visually resemble a reverse pyramid.

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Understanding the Pattern

Let’s analyze the output for N = 5.

Row	Leading Spaces	Number of Stars
1	0	5
2	1	4
3	2	3
4	3	2
5	4	1

From this table we can derive:

Leading Spaces

spaces = rowNumber

Number of Stars

stars = N - rowNumber

where row numbering starts from 0.

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Java Solution

public class ReversePyramid {

    public static void main(String[] args) {

        int n = 5;

        for (int row = 0; row < n; row++) {

            // Print leading spaces
            for (int space = 0; space < row; space++) {
                System.out.print(" ");
            }

            // Print stars separated by spaces
            for (int star = 0; star < n - row; star++) {
                System.out.print("* ");
            }

            System.out.println();
        }
    }
}

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Dry Run

Row 0

spaces = 0
stars = 5

Output:

* * * * *

Row 1

spaces = 1
stars = 4

Output:

 * * * *

Row 2

spaces = 2
stars = 3

Output:

  * * *

And so on until the last row.

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Algorithm

1. Read input integer N.
2. Iterate from row 0 to N-1.
3. Print row number of leading spaces.
4. Print (N-row) stars separated by spaces.
5. Move to the next line.
6. Repeat until all rows are printed.

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Time Complexity Analysis

Outer Loop

The outer loop executes:

N times

Inner Loops

For every row:

• Space loop executes row times.
• Star loop executes (N-row) times.

Total work per row:

row + (N-row) = N

Since each row performs approximately N operations and there are N rows:

Total Operations = N × N

Time Complexity

O(N²)

Space Complexity

O(1)

No additional data structures are used.

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Interview Discussion Points

After the candidate solves the problem, consider asking the following questions.

Question 1

Why is the time complexity O(N²)?

Expected Answer

Because for every row we perform approximately N operations and there are N rows.

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Question 2

Can you print the normal pyramid instead?

Example:

    *
   * *
  * * *
 * * * *
* * * * *

Expected Answer

Candidate should modify both the space and star calculations.

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Question 3

Can you print a diamond pattern?

Example:

    *
   * *
  * * *
 * * * *
* * * * *
 * * * *
  * * *
   * *
    *

Expected Answer

Combine a normal pyramid and a reverse pyramid.

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Question 4

Can you solve this using StringBuilder?

Expected Answer

Build each row in memory before printing to reduce repeated console writes.

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Question 5

What happens if N is negative or zero?

Expected Answer

Validate input before processing and handle invalid values appropriately.

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What This Question Evaluates

This simple-looking problem helps interviewers evaluate multiple skills:

• Nested loop understanding
• Pattern recognition
• Logical reasoning
• Dry-run capability
• Mathematical thinking
• Time complexity analysis
• Code readability