mockline

Practice the interview, not the setup.

Coding problems and system design boards that are ready the second you open them. Work alone, or with a guide watching and a voice on the line.

Invite-only while we’re small. We add people a few dozen at a time.

Two Sum II Arrays, two pointers Guide on, mic open 24:10 left

1def two_sum(numbers, target):
2    for i in range(len(numbers)):
3        for j in range(i + 1, len(numbers)):
4            if numbers[i] + numbers[j] == target:
5                return [i + 1, j + 1]
6
7    return []
8

Python 3.12 Last run: 3 of 3 samples passed, 0.9 ms

A guide that watches, and talks, while you work

Turn on the live guide and it follows along as you code, the way a good interviewer sitting next to you would. When you drift, it says so, out loud and briefly, then lets you get on with it. Answer it, ask for a hint, or keep typing.

It talks, it never types: Questions and hints by voice. The code stays yours, which is the point of practicing.
You set how much it says: From quiet until you ask, to interviewer mode where it pushes back on every choice.
It sees the problem, not your desktop: It reads what’s in the editor and the board, and hears you only while the mic is open.
Off is really off: One switch closes the mic and stops sending your work to a model. We keep notes from the session, never the audio.

Coding practice that opens instantly and measures honestly

Pick a track or a timed set. Run checks the samples while you think; Submit runs the hidden suite and times your code on fixed hardware, startup excluded.

Tracks Timed set: 45 min, 2 problems

Problem	Difficulty	Your best	Growth	Last tried
Contains duplicate	Easy	4.2 ms, top 20%	O(n)	Monday
Group anagrams	Medium	31 ms, top 45%	O(n·k log k)	Monday
Two Sum II	Medium	Samples only	O(n²), can be O(n)	In progress
Longest consecutive sequence	Medium	Not yet
Top K frequent elements	Medium	Not yet

Runtime you can compare. Every submission is timed on the same dedicated cores, several runs, median kept, compile and startup left out.

Growth, measured. We run your solution on growing inputs and tell you how it scales, which is what the interviewer will ask.

Timed sets. Two problems, 45 minutes, no hints unless you ask. The closest thing to the real thing without a stranger on the call.

Draw the system, then turn up the traffic

Sketch the architecture on the board, choose a load profile, and watch where requests queue. You find the bottleneck before the interviewer asks about it.

Results are instant and the same every time, so you can change one thing and see what it did.

Clients

Load balancer

API, 3 nodes

Redis

Postgres 96% busy, queuing

Traffic 12,000 requests per second

p95 latency 380 ms errors 0.4% bottleneck writes to Postgres

On a call: design a URL shortener 18:42

Interviewer Two users shorten the same long URL at the same moment. What do they each get back?

You The same code, if I hash the URL. But then I have to handle collisions, so maybe a counter with base62 instead.

Interviewer Fine. Where does that counter live, and what happens when that machine goes down?

When you want the pressure, pick up the call

Start a voice call with an interviewer that asks follow-ups, pushes on the weak parts of your answer, and gives you room to think. Switch to text whenever you like.

Calls are optional. What we keep afterwards is a short set of notes for your feedback, not the recording.

See what is actually getting better

Every session is scored on the same six things, with a line of evidence for each score. After a few weeks you know where you are strong and what to practice next, and so do we.

Last 8 sessions Example report

Communication3.1 to 4.2

Complexity analysis3.4 to 4.0

Trade-off reasoning2.6 to 3.5

Edge cases2.2 to 2.4, practice this

Last session: missed the empty-input case and didn’t check for duplicate keys.

Delete means delete

Practicing means getting things wrong, often. You should be able to do that without it following you around.

Notes, not recordings: We keep the scores and short notes your progress report needs. Not your voice, not full transcripts.
One switch, everything: Delete your account and your code, notes and history go with it, backups included.
You can read it all: Every note we keep about you is in your account, and you can dispute it.