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CodePeet 444 Complete DSA Sheet

Right-Angled Triangle of Stars

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EasyProblem

Description

Given n, return a right-angled triangle of stars, n rows high, as a list of rows: the first row has 1 star, the second 2, and so on, until the last row has n.

Examples

Example 1

Input: n = 3

Output: ["", "", ""]

Explanation: Row 1 has one star, row 2 two, row 3 three.

Example 2

Input: n = 1

Output: ["*"]

Explanation: One row with one star.

Example 3

Input: n = 5

Output: ["", "", "", "****", "*****"]

Explanation: Five rows, growing by one star each.

Constraints

  • 1 ≤ n ≤ 100

Editorial

Approach - Build Each Row from Its Counts

Intuition

Row i, counting from 1, is just i stars. So one loop over the rows, from 1 to n, builds each row from its count: "*" * i in Python, "*".repeat(i) in Java and JavaScript, string(i, '*') in C++.

Step-by-Step Explanation

Let's trace Example 1, n = 3:

  1. Row 1: 1 star.
  2. Row 2: 2 stars.
  3. Row 3: 3 stars.
  4. Return the 3 rows.

Right-Angled Triangle of Stars — The rows, built one per step from the top; the row just built is lit.

Algorithm

  1. For i from 1 to n, add a row of i stars.
  2. Return the rows.

Code

class Solution {
public:
    vector<string> rightTriangle(int n) {
        vector<string> rows;
        for (int i = 1; i <= n; i++) rows.push_back(string(i, '*'));   // row i has i stars
        return rows;
    }
};

from typing import List

class Solution:
    def rightTriangle(self, n: int) -> List[str]:
        return ["*" * i for i in range(1, n + 1)]      # row i has i stars

import java.util.*;

class Solution {
    public List<String> rightTriangle(int n) {
        List<String> rows = new ArrayList<>();
        for (int i = 1; i <= n; i++) rows.add("*".repeat(i));   // row i has i stars
        return rows;
    }
}

var rightTriangle = function(n) {
    const rows = [];
    for (let i = 1; i <= n; i++) rows.push("*".repeat(i));   // row i has i stars
    return rows;
};

Complexity Analysis

Time Complexity: O(n²)

There are O(n) rows of O(n) characters, and each character is written once.

Space Complexity: O(n²)

The rows of the answer.

Common Mistakes

Each of these looks right on a quick read, and the tests still reject it. The outputs below come from running the code.

Counting rows from 0. range(n)

On n = 3 it returns ["", "*", "**"]; the answer is ["*", "**", "***"]. range(n) starts at 0, so the first row has 0 stars, an empty string, and the last has n - 1. The rows run from 1 to n: range(1, n + 1).

Stopping one row short. range(1, n)

On n = 3 it returns ["*", "**"]; the answer is ["*", "**", "***"]. range(1, n) stops before n, so the last row, with n stars, is missing.

Building it upside down. "*" * (n - i + 1)

On n = 3 it returns ["***", "**", "*"]; the answer is ["*", "**", "***"]. n - i + 1 stars puts the longest row first. The triangle grows downwards: row i has i stars.

Where This Pattern Is Taught

This problem practises nested counting: for each row, how many of each character, and building the row as a string. It's taught in Python for DSA, Java for DSA, C++ for DSA and JavaScript for DSA.