What is a practical coding interview?

A round where you build a small working program instead of solving one algorithm puzzle: a parser, a class with several methods that share state, a store with expiry or transactions. It usually comes in parts, and each part adds a requirement to the code you already wrote, so the interviewer sees how your design holds up under change.

Where do these problems come from?

The problems themselves are original, written around the patterns candidate reports mention most. What comes from reports is the progression: each page lists the follow-ups candidates said interviewers added on questions with that pattern, with how many reports mention each and at which companies, and the parts marked "from reports" are built on them. The company guides list the questions candidates described.

Is LeetCode enough for coding interviews in 2026?

It covers part of it. 7 of the 10 most common topics in 1,447 candidate reports are practical coding, such as stateful class design and parsing, which classic LeetCode lists cover lightly. BFS, trees, hash maps and heaps still matter, usually as one step inside a larger problem.

How should I use these problems?

Set a timer for the listed minutes, ask yourself the clarifying questions first, and solve part 1 completely before reading part 2. Run the tests after each part. Afterwards compare with the reference solution and read the follow-ups, then try one out loud as if an interviewer had asked it.

Practical coding interview examples

Coding rounds have moved past one-shot puzzles. In 1,447 interview reports from 18 tech companies, the most common tasks were building a class whose methods share state, and parsing messy input. These 12 problems train those patterns. Each starts from the standard version, then adds the follow-ups candidates reported interviewers asking, as parts that extend the same code. Tests run in your browser, with hints, a walkthrough and a reference solution. Free, no account.

Which topics, and why: coding interview topics ranked by candidate reports

Stateful class design

19.7% of reports · 16 companies

Parking Lot Design

Medium · 35 min

Design a multi-level parking lot class: park vehicles in spots that fit their size, then choose the smallest fitting spot quickly and look up where a vehicle is, then report the busiest period.

Park and leave → Best-fit spot and lookup → Peak occupancy

5 follow-ups from interview reports

Also: Heaps, top-k and greedy scheduling

Parsing structured text

18.5% of reports · 15 companies

Log File Parser

Medium · 45 min

Parse web server log lines while counting malformed ones, compute per-endpoint request, error and latency stats, find the busiest endpoints in a time window, then answer the same questions over a live stream.

Parse the lines → Per-endpoint stats → Busiest endpoints in a time window → Live stream with a sliding window

5 follow-ups from interview reports

Also: Hash-map counting, grouping and joins, Sorting with custom comparators

BFS shortest path (grids and state graphs)

12.6% of reports · 12 companies

Shortest Path in a Grid (BFS)

Medium · 50 min

Find the fewest moves between two cells of a grid with walls, return the path itself, then handle the two twists candidates report most: a battery that charging cells refill, and a grid with no edges.

Path length → Return the path → Limited battery → No grid edges

5 follow-ups from interview reports

Also: Simulation and game rules

Tree recursion and traversal

12.2% of reports · 13 companies

Lowest Common Manager (Org Chart)

Medium · 40 min

Build an org chart from (employee, manager) pairs, count reports and walk the chain of command, find the lowest common manager of two or more people, then support moving people around and asking who sits above whom.

Reports and chain of command → Lowest common manager → Moves and relationship checks

5 follow-ups from interview reports

Also: Hash-map counting, grouping and joins, DFS and connected components

Hash-map counting, grouping and joins

10.5% of reports · 14 companies

CSV Join and Group By

Medium · 45 min

Parse CSV text into rows, sum a column per group, left join two tables on a key column with missing matches, then add the filter and sort queries candidates report most: WHERE conditions and multi-column ORDER BY.

Group by and sum → Left join → Filter and sort

5 follow-ups from interview reports

Also: Sorting with custom comparators, Parsing structured text

Time-based state (expiry, TTL, snapshots)

10.4% of reports · 9 companies

Time-Based Key-Value Store with TTL

Medium · 45 min

Design a key-value store whose keys expire after a time-to-live, with time passed in. Then renew keys and count live ones efficiently, look up past values, and back up and restore the store.

Set and get with expiry → Renew keys and count the live ones → Read the past → Backup and restore

5 follow-ups from interview reports

Also: Hash-map counting, grouping and joins, In-memory database or key-value store

In-memory database or key-value store

9.1% of reports · 8 companies

In-Memory Key-Value Store with Transactions

Medium · 35 min

Build an in-memory key-value store with set, get, delete and count-by-value, then nested transactions with begin, rollback and commit, then a sorted prefix scan.

Basic operations → Nested transactions → Prefix scan

5 follow-ups from interview reports

Also: Stateful class design

Sliding window, two pointers, string matching

6.8% of reports · 10 companies

Design Hit Counter

Medium · 40 min

Count the hits in the last 300 seconds, then per key, then over any window length, then at very high hit rates without storing one entry per hit.

Count hits in the last 300 seconds → Counts per key → Any window length → Very high hit rates

5 follow-ups from interview reports

Also: Hash-map counting, grouping and joins, Time-based state (expiry, TTL, snapshots)

Intervals and sweep lines

6.5% of reports · 10 companies

Meeting Rooms II: Room Booking

Medium · 50 min

Book meetings into one room without overlaps, assign the lowest-numbered free room and count how many rooms a schedule needs, add cancellation and an earliest-free-slot search, then report the busiest periods.

One room → Many rooms → Cancel and find a free slot → Busiest periods

5 follow-ups from interview reports

Also: Heaps, top-k and greedy scheduling

Rate limiting and time-window counts

6.1% of reports · 10 companies

Rate Limiter

Medium · 45 min

Build a fixed-window rate limiter, replace it with a sliding window that closes the boundary burst, then enforce several rules at once and implement a token bucket.

Fixed window → Sliding window → Several rules at once → Token bucket

5 follow-ups from interview reports

Also: Sliding window, two pointers, string matching, Time-based state (expiry, TTL, snapshots)

Expression evaluation and interpreters

6.1% of reports · 8 companies

Basic Calculator (Expression Evaluator)

Medium · 50 min

Evaluate an integer arithmetic string with + - * /, parentheses and unary minus, then nested add/sub/mul/div calls, then variables defined by other expressions, then raise ValueError on bad input.

Arithmetic with precedence → Nested function calls → Variables defined by expressions → Invalid input

5 follow-ups from interview reports

Also: Parsing structured text, Stacks (brackets, call stacks, monotonic stacks)

LRU and LFU caches

1.7% of reports · 6 companies

LRU Cache

Medium · 35 min

Build the classic O(1) least recently used cache, then add per-item expiry with time passed in and a capacity measured in total weight.

LRU cache → Per-item expiry → Weighted capacity

5 follow-ups from interview reports

Also: Stateful class design, Time-based state (expiry, TTL, snapshots)

Frequently asked

What is a practical coding interview?: A round where you build a small working program instead of solving one algorithm puzzle: a parser, a class with several methods that share state, a store with expiry or transactions. It usually comes in parts, and each part adds a requirement to the code you already wrote, so the interviewer sees how your design holds up under change.
Where do these problems come from?: The problems themselves are original, written around the patterns candidate reports mention most. What comes from reports is the progression: each page lists the follow-ups candidates said interviewers added on questions with that pattern, with how many reports mention each and at which companies, and the parts marked "from reports" are built on them. The company guides list the questions candidates described.
Is LeetCode enough for coding interviews in 2026?: It covers part of it. 7 of the 10 most common topics in 1,447 candidate reports are practical coding, such as stateful class design and parsing, which classic LeetCode lists cover lightly. BFS, trees, hash maps and heaps still matter, usually as one step inside a larger problem.
How should I use these problems?: Set a timer for the listed minutes, ask yourself the clarifying questions first, and solve part 1 completely before reading part 2. Run the tests after each part. Afterwards compare with the reference solution and read the follow-ups, then try one out loud as if an interviewer had asked it.

Preparing for a specific company?

The company guides collect what candidates report at each company: the rounds, the questions, their parts and follow-ups, and how often each one comes up.

Browse the interview guides