ALX Interview — Technical Preparation

A curated collection of 11 algorithmic and systems challenges designed to sharpen the problem-solving skills tested in technical interviews. Each challenge lives in its own directory with a reference implementation, main test driver, and focused README.

These problems span Python, JavaScript, and core computer-science topics: dynamic programming, graph traversal, backtracking, bit manipulation, HTTP APIs, and stream processing.

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Skills covered

• Translate ambiguous interview prompts into precise algorithm designs
• Choose appropriate data structures (arrays, graphs, stacks, hash maps) under time pressure
• Build solutions that handle edge cases and invalid input gracefully
• Analyze time and space complexity and articulate trade-offs verbally
• Work with standard libraries (re, request, sys) and external APIs
• Write clean, PEP 8–compliant Python and idiomatic Node.js

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Challenge Index

#	Directory	Problem	Core Concepts	Language
0	0x00-pascal_triangle	Generate Pascal's triangle to n rows	2D arrays, combinatorics	Python
1	0x01-lockboxes	Determine if all lockboxes can be opened	Graph reachability, BFS/stack	Python
2	0x02-minimum_operations	Minimum copy/paste ops to get n H characters	Number theory, prime factorization	Python
3	0x03-log_parsing	Parse stdin log lines, aggregate stats	Regex, signal handling, streaming I/O	Python
4	0x04-utf8_validation	Validate a byte sequence as UTF-8	Bit masking, encoding rules	Python
5	0x05-nqueens	Solve the N-Queens puzzle	Backtracking, constraint satisfaction	Python
6	0x06-starwars_api	Print Star Wars film characters in order	HTTP requests, async recursion, JSON	JavaScript
7	0x07-rotate_2d_matrix	Rotate an n×n matrix 90° clockwise in-place	Matrix transpose, in-place algorithms	Python
8	0x08-making_change	Fewest coins to make a target amount	Dynamic programming, coin change	Python
9	0x09-island_perimeter	Compute perimeter of a grid island	Grid traversal, adjacency counting	Python
10	0x0A-primegame	Determine winner of a prime-selection game	Game theory, sieve, optimal play	Python

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Challenge Summaries

0x00. Pascal's Triangle

Implement pascal_triangle(n) returning the first n rows of Pascal's triangle. Each interior element is the sum of the two elements above it. Edge case: return [] when n ≤ 0.

Skills: Nested loops, list comprehensions, mathematical patterns.

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0x01. Lockboxes

Given n boxes where box i contains keys to other boxes, determine whether every box can be opened starting from box 0 (which is unlocked). Model keys as edges in a directed graph and check reachability.

Skills: Graph modeling, stack-based DFS, boolean tracking arrays.

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0x02. Minimum Operations

Starting with a file containing one H, you may only Copy All then Paste. Find the minimum operations to produce exactly n H characters. The optimal strategy relates to the prime factorization of n.

Skills: Divisor iteration, greedy decomposition, O(√n) analysis.

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0x03. Log Parsing

Read Apache-style log lines from stdin in real time. Every 10 lines (and on SIGINT), print total transferred size and counts per HTTP status code (200, 301, 400, 401, 403, 404, 405, 500). Uses regex to parse each line.

Skills: Regular expressions, signal handlers, incremental aggregation, Unix pipelines.

./0-generator.py | ./0-stats.py

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0x04. UTF-8 Validation

Given a list of integers representing bytes, return True if they form a valid UTF-8 encoding. Handle 1-byte ASCII, 2-byte, 3-byte, and 4-byte sequences using bit-level checks.

Skills: Bitwise operations, state machine over byte sequences.

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0x05. N Queens

Place N non-attacking queens on an N×N board. Print every solution as a list of [row, column] coordinates. N must be an integer ≥ 4.

Skills: Backtracking, diagonal/column conflict detection, recursion.

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0x06. Star Wars API

Node.js script that accepts a Star Wars film ID, fetches the film from the SWAPI-compatible endpoint, then retrieves and prints each character's name in the order listed in the film response.

Skills: HTTP with request, nested async callbacks, JSON parsing, CLI arguments.

./0-starwars_characters.js 3

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0x07. Rotate 2D Matrix

Rotate an n×n integer matrix 90 degrees clockwise in place (no return value). Achieve this with transpose + row reversal.

Skills: In-place matrix manipulation, index swapping.

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0x08. Making Change

Given coin denominations and a target total, return the minimum number of coins needed, or -1 if impossible. Classic bottom-up dynamic programming.

Skills: DP table construction, unreachable-state handling.

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0x09. Island Perimeter

Given a 2D grid of 0 (water) and 1 (land), compute the perimeter of the single island. Count exposed edges; no lakes inside the island.

Skills: Grid iteration, boundary detection, adjacency logic.

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0x0A. Prime Game

Maria and Ben alternate removing a prime and all its multiples from a set {1, …, x} across multiple rounds. Both play optimally; Maria goes first. Return the overall winner ("Maria", "Ben", or None on tie).

Skills: Prime sieving, combinatorial game theory, simulation.

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How to Run

Each challenge directory is self-contained:

# Python challenges
cd 0x00-pascal_triangle
python3 0-main.py

# JavaScript challenge
cd 0x06-starwars_api
./0-starwars_characters.js 1

Most Python tasks include a *-main.py driver that imports and exercises the solution function with predefined test cases matching the ALX checker format.

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Recommended Study Order

1. Warm-up — Pascal's Triangle, Rotate 2D Matrix, Island Perimeter
2. Core algorithms — Lockboxes, Minimum Operations, Making Change, N Queens
3. Systems & I/O — Log Parsing, UTF-8 Validation, Star Wars API
4. Advanced — Prime Game (combines number theory and game theory)

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Environment

Tool	Version
Python	3.x
Node.js	For 0x06-starwars_api
request npm module	Star Wars API challenge

No external dependencies are required for the Python-only challenges beyond the standard library.

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License

Public domain.