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easyBasicsSometimes asked

Square Star Pattern

Basics · 6 min

Given a positive integer n, return an n × n square of *, rows separated by newlines.

In plain English

Nested loop where every row has exactly n stars.

Examples

Example 1

Input: n = 2
Output: "**\n**"
Note: 2x2 block.

Example 2

Input: n = 3
Output: "***\n***\n***"
Note: 3x3 block.

Example 3

Input: n = 1
Output: "*"
Note: Single star.

Constraints

1 ≤ n ≤ 1000

Clarifying questions1First question: Filled or hollow square?Explore questions

Q1.Filled or hollow square?
Filled here; hollow would print stars only on the border.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Outer loop n rows; inner loop appends n stars each; newline between rows.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you keep the inner bound constant for a rectangle/square.

Where candidates slip

Accidentally indexing the inner bound by the row.

How to stand out

Contrast with the triangle to show you understand the bound's role.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Nested loop

TimeO(n²)SpaceO(n²)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

How does the inner bound differ from the triangle's?

2Hint 2Explore hint

Both loops run n times — a constant inner bound.

3Hint 3Explore hint

Newlines between rows only.

A · Core idea

The simplest nested pattern: both dimensions are fixed at n, so every one of the n rows has exactly n stars.

Why a constant inner bound. Unlike the triangle (inner bound = row index), a square's inner loop always runs n times. Getting this constant bound right — not accidentally making it triangular — is the whole exercise.

Why O(n²). n rows × n columns = n² characters.

Newline discipline. Separate rows with a newline placed between them, not trailing the last row.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

n = 2

row 1

2 stars

State**

Final result"**\n**"

C · Code with teaching comments

solution.javaJava

Nested loop — optimal

1
public String square(int n) {
2
    StringBuilder sb = new StringBuilder();
3
    for (int i = 0; i < n; i++) {
4
        for (int j = 0; j < n; j++) sb.append('*');
5
        if (i < n - 1) sb.append('\n');
6
    }
7
    return sb.toString();
8
}

D · Why it works and complexityO(n²) timeO(n²) spaceOpen the invariant and proofExplore the proof

ComplexityO(n²) time, O(n²) space — n rows each emitting n characters → n² total.

E · Edge cases and pitfalls4n = 1 — "*".Explore edge cases

Edge cases handled

n = 1"*".
n = 33×3 block.

Pitfalls

Using a triangular inner bound by accident.
Trailing newline.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Editable code previewpublic String square(int n) { StringBuilder sb = new StringBuilder(); for (int i = 0; i < n; i++) { for (int j = 0; j < n; j++) sb.append('*'); if (i < n - 1) sb.append('\n');Explore the playground

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Using a triangular inner bound by accident.
Trailing newline.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Fixed nested loop

Code pattern to memorize

for i in 1..n: print('*' * n)

Rules to remember

1n rows × n cols.
2Constant inner bound.
3Newlines between rows.

Use this pattern when

Rectangular patterns

Interview takeaway

A square is the nested loop with both bounds equal to n.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: Fixed nested loop. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Right Triangle Star
Make the inner bound the row index.

easyBasicsSometimes asked

Square Star Pattern

Basics · 6 min

Given a positive integer n, return an n × n square of *, rows separated by newlines.

In plain English

Nested loop where every row has exactly n stars.

Examples

Example 1

Input: n = 2
Output: "**\n**"
Note: 2x2 block.

Example 2

Input: n = 3
Output: "***\n***\n***"
Note: 3x3 block.

Example 3

Input: n = 1
Output: "*"
Note: Single star.

Constraints

1 ≤ n ≤ 1000

Clarifying questions1First question: Filled or hollow square?Explore questions

Q1.Filled or hollow square?
Filled here; hollow would print stars only on the border.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Outer loop n rows; inner loop appends n stars each; newline between rows.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you keep the inner bound constant for a rectangle/square.

Where candidates slip

Accidentally indexing the inner bound by the row.

How to stand out

Contrast with the triangle to show you understand the bound's role.

Choose

Compare the available approaches

Use constraints and trade-offs to select the route you can defend in an interview.

Learn & trace

Build each solution step by step

Move from the core idea to a visual trace, implementation, proof, and edge cases.

03.1

Approach 1 of 1 · Optimal

Nested loop

TimeO(n²)SpaceO(n²)

This is the standard, expected solution — lead with it.

Stuck? Progressive hintsReveal one at a time

1Hint 1Explore hint

How does the inner bound differ from the triangle's?

2Hint 2Explore hint

Both loops run n times — a constant inner bound.

3Hint 3Explore hint

Newlines between rows only.

A · Core idea

The simplest nested pattern: both dimensions are fixed at n, so every one of the n rows has exactly n stars.

Why O(n²). n rows × n columns = n² characters.

Newline discipline. Separate rows with a newline placed between them, not trailing the last row.

B · Visual dry run

Interactive dry run

Select a frame to connect the decision with its visual state.

n = 2

row 1

2 stars

State**

Final result"**\n**"

C · Code with teaching comments

solution.javaJava

Nested loop — optimal

1
public String square(int n) {
2
    StringBuilder sb = new StringBuilder();
3
    for (int i = 0; i < n; i++) {
4
        for (int j = 0; j < n; j++) sb.append('*');
5
        if (i < n - 1) sb.append('\n');
6
    }
7
    return sb.toString();
8
}

D · Why it works and complexityO(n²) timeO(n²) spaceOpen the invariant and proofExplore the proof

ComplexityO(n²) time, O(n²) space — n rows each emitting n characters → n² total.

E · Edge cases and pitfalls4n = 1 — "*".Explore edge cases

Edge cases handled

n = 1"*".
n = 33×3 block.

Pitfalls

Using a triangular inner bound by accident.
Trailing newline.

Practice

Try the solution yourself

Edit the optimal implementation, test another input, and observe the result.

Try it — Java· run a complete program; method snippets need a driver

Avoid mistakes

Recognize the interview traps

Understand why common attempts fail before reviewing the corrected implementation.

Using a triangular inner bound by accident.
Trailing newline.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Fixed nested loop

Code pattern to memorize

for i in 1..n: print('*' * n)

Rules to remember

1n rows × n cols.
2Constant inner bound.
3Newlines between rows.

Use this pattern when

Rectangular patterns

Interview takeaway

A square is the nested loop with both bounds equal to n.

Continue

Transfer the pattern to another problem

Use a direct variation or pattern sibling to prove the idea is reusable.

Transferable patternA core fresher pattern: Fixed nested loop. Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

Right Triangle Star
Make the inner bound the row index.