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easyBasicsSometimes asked

Right Triangle Star Pattern

Basics · 6 min

Given a positive integer n, return a right-angled triangle of * with i stars on row i, rows separated by newlines.

In plain English

Nested loop: outer row 1..n, inner prints that many stars.

Examples

Example 1

Input: n = 3
Output: "*\n**\n***"
Note: 1, 2, 3 stars.

Example 2

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

Example 3

Input: n = 4
Output: "*\n**\n***\n****"
Note: Four rows.

Constraints

1 ≤ n ≤ 1000

Clarifying questions1First question: Left- or right-aligned?Explore questions

Q1.Left- or right-aligned?
Left-aligned right triangle here; right-aligned adds leading spaces.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Outer loop over rows 1..n; inner loop appends `i` stars; add a newline between rows.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you can structure nested loops with an index-dependent inner bound.

Where candidates slip

Off-by-one star count, or trailing newline.

How to stand out

State the row/column mental model for all pattern problems.

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

A 2-D shape needs two loops — what does each control?

2Hint 2Explore hint

Row i should have how many stars?

3Hint 3Explore hint

Place newlines between rows, not after the last.

A · Core idea

The introduction to nested loops: the outer loop selects the row, the inner loop draws that row's characters. Here row i contains i stars.

Why nested loops. A 2-D shape needs two indices — one for rows, one for columns. The inner loop's bound depends on the outer index (i stars on row i), which is what makes it a triangle rather than a rectangle.

Why O(n²). Row i does i appends, so total work is 1+2+…+n = n(n+1)/2 = O(n²) — inherent to drawing the shape.

Newline discipline. Put the newline between rows (e.g. if (i < n)), not after the last, to avoid a trailing blank line.

B · Visual dry run

Interactive dry run

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

n = 3

row 1

1 star

State*

Final result"*\n**\n***"

C · Code with teaching comments

solution.javaJava

Nested loop — optimal

1 teaching note

1
public String triangle(int n) {
2
    StringBuilder sb = new StringBuilder();
3
    for (int i = 1; i <= n; i++) {
4
        // Row i gets i stars.
5
        for (int j = 0; j < i; j++) sb.append('*');
6
        if (i < n) sb.append('\n');
7
    }
8
    return sb.toString();
9
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.9 lines

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

ComplexityO(n²) time, O(n²) space — row i emits i characters, so total output is 1+2+…+n = O(n²).

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

Edge cases handled

n = 1"*".
n = 4Four rows, 1..4 stars.

Pitfalls

Inner loop bound wrong (i+1 or i-1 stars).
Trailing newline after the last row.

Practice

Try the solution yourself

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

Editable code previewpublic String triangle(int n) { StringBuilder sb = new StringBuilder(); for (int i = 1; i <= n; i++) { for (int j = 0; j < i; j++) sb.append('*'); if (i < n) 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.

Inner loop bound wrong (i+1 or i-1 stars).
Trailing newline after the last row.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Nested loop (rows × cols)

Code pattern to memorize

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

Rules to remember

1Outer = rows.
2Inner = count per row.
3Newlines between rows.

Use this pattern when

Any row/column pattern

Interview takeaway

Patterns = outer loop for rows, inner loop for columns.

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: Nested loop (rows × cols). Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups

easyBasicsSometimes asked

Right Triangle Star Pattern

Basics · 6 min

Given a positive integer n, return a right-angled triangle of * with i stars on row i, rows separated by newlines.

In plain English

Nested loop: outer row 1..n, inner prints that many stars.

Examples

Example 1

Input: n = 3
Output: "*\n**\n***"
Note: 1, 2, 3 stars.

Example 2

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

Example 3

Input: n = 4
Output: "*\n**\n***\n****"
Note: Four rows.

Constraints

1 ≤ n ≤ 1000

Clarifying questions1First question: Left- or right-aligned?Explore questions

Q1.Left- or right-aligned?
Left-aligned right triangle here; right-aligned adds leading spaces.

Quick interview revision

Understand

30-second explanation

Build the interview-ready mental model before comparing implementations.

Outer loop over rows 1..n; inner loop appends `i` stars; add a newline between rows.

Interview contextWhat is testedWhere candidates slipHow to stand outExplore interview context

What's being tested

Whether you can structure nested loops with an index-dependent inner bound.

Where candidates slip

Off-by-one star count, or trailing newline.

How to stand out

State the row/column mental model for all pattern problems.

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

A 2-D shape needs two loops — what does each control?

2Hint 2Explore hint

Row i should have how many stars?

3Hint 3Explore hint

Place newlines between rows, not after the last.

A · Core idea

The introduction to nested loops: the outer loop selects the row, the inner loop draws that row's characters. Here row i contains i stars.

Why O(n²). Row i does i appends, so total work is 1+2+…+n = n(n+1)/2 = O(n²) — inherent to drawing the shape.

Newline discipline. Put the newline between rows (e.g. if (i < n)), not after the last, to avoid a trailing blank line.

B · Visual dry run

Interactive dry run

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

n = 3

row 1

1 star

State*

Final result"*\n**\n***"

C · Code with teaching comments

solution.javaJava

Nested loop — optimal

1 teaching note

1
public String triangle(int n) {
2
    StringBuilder sb = new StringBuilder();
3
    for (int i = 1; i <= n; i++) {
4
        // Row i gets i stars.
5
        for (int j = 0; j < i; j++) sb.append('*');
6
        if (i < n) sb.append('\n');
7
    }
8
    return sb.toString();
9
}

Yellow row = the algorithm's core logic. Amber `//` lines above it = what you'd say out loud.9 lines

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

ComplexityO(n²) time, O(n²) space — row i emits i characters, so total output is 1+2+…+n = O(n²).

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

Edge cases handled

n = 1"*".
n = 4Four rows, 1..4 stars.

Pitfalls

Inner loop bound wrong (i+1 or i-1 stars).
Trailing newline after the last row.

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.

Inner loop bound wrong (i+1 or i-1 stars).
Trailing newline after the last row.

Revise

Carry the pattern into the interview

Quick interview revision

Pattern to remember

Nested loop (rows × cols)

Code pattern to memorize

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

Rules to remember

1Outer = rows.
2Inner = count per row.
3Newlines between rows.

Use this pattern when

Any row/column pattern

Interview takeaway

Patterns = outer loop for rows, inner loop for columns.

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: Nested loop (rows × cols). Master it on small inputs and it transfers to harder problems.

Practice queueDirect variations and pattern siblings

Direct follow-ups