coding-interview-pattern-drill

Why this exists

LeetCode-style interviews are a poor proxy for real engineering ability. They favour people who've memorised solutions over people who can reason through novel problems, and they routinely ask you to reimplement things that exist perfectly well in the standard library. They're also ubiquitous and unavoidable, so here we are.

The biggest source of wasted time in LeetCode prep is misidentifying the pattern. You read a problem, reach for the wrong tool — BFS when you needed Dijkstra, DP when it's just Greedy — and burn 20 minutes going nowhere. The actual coding, once you know the approach, usually takes 10 minutes.

coding-interview-pattern-drill targets that gap. Given a real problem description, pick the right algorithmic pattern from four options before writing a single line of code. Wrong options are drawn from a hand-tuned confusion map — the patterns most commonly mistaken for the correct one — so the choices are meaningfully hard rather than obviously wrong.

The goal is to build fast, confident pattern recognition so that in an interview you spend your time on implementation, not on figuring out where to start.

Problems are pulled from the LeetCode API and presented as multiple-choice questions. Wrong options come from a hand-tuned confusion map (the patterns most commonly mistaken for the right one), not random noise.

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Installation

Homebrew (macOS / Linux)

brew tap conallob/taps
brew install coding-interview-pattern-drill

Go install

go install github.com/conallob/coding-interview-pattern-drill/cmd/coding-interview-pattern-drill@latest

Build from source

git clone https://github.com/conallob/coding-interview-pattern-drill.git
cd coding-interview-pattern-drill
go build -o coding-interview-pattern-drill ./cmd/coding-interview-pattern-drill

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Authentication

The tool fetches problems from the LeetCode API, which requires a valid session cookie.

1. Log in to leetcode.com in your browser
2. Open DevTools (F12 or ⌘⌥I)
3. Go to Application → Cookies → https://leetcode.com
4. Copy the value of the LEETCODE_SESSION cookie

You can supply it in two ways:

Environment variable (CLI mode):

export LEETCODE_SESSION=<paste value here>
export LEETCODE_CSRF=<csrftoken value, if required>

Browser UI (server mode): paste it into the settings form — no terminal required. See HTTP Server mode below.

Credentials saved via the browser UI are persisted to ~/.config/pattern-drill/credentials.json (mode 0600) and survive restarts.

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CLI mode

Run a quiz session directly in your terminal.

coding-interview-pattern-drill [flags]

Flags

Flag	Default	Description
--difficulty <list>	(all)	Comma-separated difficulties to include: easy,medium,hard
--tag <list>	(all patterns)	Comma-separated pattern slugs to include (see --list-tags)
--count N	10	Number of problems to quiz
--list-tags	—	Print all 18 pattern slugs and exit
--refresh-cache	—	Force a fresh fetch from LeetCode, ignoring the local cache

Examples

# Quick 10-question session across all difficulties and patterns
coding-interview-pattern-drill

# 20 medium problems only
coding-interview-pattern-drill --difficulty medium --count 20

# Easy and hard, but not medium
coding-interview-pattern-drill --difficulty easy,hard

# Drill two commonly-confused patterns together
coding-interview-pattern-drill --tag dynamic-programming,backtracking

# Medium/hard problems from graph-related patterns only
coding-interview-pattern-drill --difficulty medium,hard --tag depth-first-search,breadth-first-search

# See all available pattern slugs
coding-interview-pattern-drill --list-tags

# Force refresh the problem list
coding-interview-pattern-drill --refresh-cache

Quiz flow

─────────────────────────────────────────────────────────────────
Question 3 of 10   Medium

Longest Substring Without Repeating Characters

Given a string s, find the length of the longest substring
without repeating characters.

  Input: s = "abcabcbb"
  Output: 3

What algorithmic pattern does this problem use?

  [A] Dynamic Programming
  [B] Sliding Window
  [C] Hash Map
  [D] Two Pointers

(a/b/c/d or q to quit)

After answering:

✓ Correct! The answer is Sliding Window.
  Also tagged: Hash Map

Type q at any prompt to quit early. A final score is shown at the end of each session.

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HTTP server mode

Starts a local web server and opens the quiz in your browser. Cookie management happens entirely in the browser — no environment variables needed.

coding-interview-pattern-drill serve [flags]

Flags

Flag	Default	Description
--port N	7777	Port to listen on
--no-open	—	Don't auto-open the browser

Examples

# Start on the default port and open the browser
coding-interview-pattern-drill serve

# Use a custom port without opening the browser
coding-interview-pattern-drill serve --port 8080 --no-open

Then open http://localhost:7777 (or your chosen port) if it didn't open automatically.

First-time setup in the browser

1. Click the ⚙ Settings gear icon
2. Paste your LEETCODE_SESSION cookie value into the field
3. Click Save — credentials are stored locally and reused on restart

Quiz configuration

From the main screen, select:

• Difficulty: All / Easy / Medium / Hard
• Pattern: All Patterns, or any of the 18 specific patterns
• Questions: how many to include in the session (default 10)

Then hit Start Quiz. Use the A B C D keys to answer, and Enter or N to advance after each result.

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Supported patterns

The tool recognises 18 algorithmic patterns. Use the slug with --tag in CLI mode.

Pattern	Slug
Two Pointers	two-pointers
Sliding Window	sliding-window
Binary Search	binary-search
Dynamic Programming	dynamic-programming
Greedy	greedy
DFS	depth-first-search
BFS	breadth-first-search
Backtracking	backtracking
Heap / Priority Queue	heap-priority-queue
Hash Map	hash-table
Stack	stack
Monotonic Stack	monotonic-stack
Topological Sort	topological-sort
Union Find	union-find
Trie	trie
Divide and Conquer	divide-and-conquer
Prefix Sum	prefix-sum
Bit Manipulation	bit-manipulation

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Caching

Problems are cached locally to avoid hammering the LeetCode API:

Cache	Location	TTL
Problem list	~/.cache/pattern-drill/problems.json	24 hours
Problem content	~/.cache/pattern-drill/content.json	No expiry

Use --refresh-cache (CLI) or the Refresh Cache button (browser UI) to force a fresh fetch.