Practice with a map, not a pile
Problems are tagged with concepts and patterns, and concepts know their prerequisites. You see what a problem teaches and what to know first.
Concept Pattern
PrivateFor you and your circle
CodeDuel: Code.Compete.Improve.
A competitive programming platform for daily practice and real contests. Every submission runs in an isolated sandbox, and a mentor helps you plan what to solve next.
- Python 3.12
- Java 21
- Sandboxed judge
- ICPC-style contests
VerdictAccepted
- Runtime
- 56 ms
- Memory
- 12.4 MB
Why CodeDuel
Practising alone is easy to quit. CodeDuel gives it direction.
CodeDuel is a private competitive programming platform. It exists so a small group can practise every day, trust their results and compete fairly, without the noise of a public site.
The problem
You solve random problems and never know what to practise next.
What CodeDuel does
Every problem is linked to the concepts it teaches, and a mentor turns your goal into a week-by-week plan.
See how practice worksThe problem
Online judges feel like a black box, and friendly contests end in arguments.
What CodeDuel does
Every run is sandboxed and ends in an exact verdict. Contests follow fixed ICPC-style rules anyone can check.
See how judging worksThe problem
Public platforms are noisy, and your progress gets lost in the crowd.
What CodeDuel does
A small, private place to practise and compete with people you know, and to see your own improvement.
See how contests work
How it works
From a goal to a verdict in four steps.
No setup. Create an account and you can be solving a problem in a minute.
- 1
Set a goal
Tell the mentor what you are aiming for. It proposes a plan, week by week, and you confirm it.
- 2
Solve in the browser
Open a problem, write Python or Java, and run it on the sample cases. Your draft is saved as you type.
- 3
Get an exact verdict
Submit. Your code runs in an isolated sandbox and every test case gets a result you can read.
- 4
Compete and improve
Join a contest with your circle, follow the standings, and watch your plan close as you solve.
Practice
Always know what to solve next, and why.
Solvespractising without direction
Most people practise by scrolling a list. CodeDuel links every problem to the concepts it teaches, so each session builds on the last.
Python 3.12 and Java 21
Write in the browser, keep a draft per problem and language, and submit when ready.
- Python 3.12draft saved
- Java 21draft saved
Results as they happen
A submission streams its progress to you while it runs, with a verdict for every test case.
Running test case 5 of 7
Built for a small, private circle
No public leaderboard to chase. CodeDuel is a place to practise and compete with people you actually know.
your circle
Judge
A verdict you can trust, whatever the code does.
Solvesnot trusting the result
Submitted code is treated as untrusted. Every run happens in its own locked-down container, and the result is an exact verdict per test case, not a guess.
- 1
Fresh container
A new, locked-down container is created for this run only.
- 2
Code copied in
Your source and the test input go in; nothing else is shared.
- 3
Run under limits
A supervisor enforces the wall clock and resource ceilings.
- 4
Measure
Exit code, signal, CPU and wall time, and peak memory are recorded.
- 5
Destroyed
The container is removed, even if the run crashed.
No network
Every run starts with networking off.
One container per run
Created, run and destroyed. Nothing is reused.
Hard resource ceilings
CPU, memory, processes and file handles are capped.
Unprivileged and read-only
Non-root, all capabilities dropped, read-only filesystem.
judge
$ judge --submission latest
ACCEPTED
Every hidden test case matched.
The runner measures
- exit code
- signal
- wall time
- CPU time
- peak memory
It never decides the outcome. A separate judging step turns these measurements into a verdict.
Contests
Friendly contests end in arguments. Not these.
Solvescontests that end in arguments
Competing with friends is only fun when everyone trusts the result. These are the usual ways it goes wrong, and what stops each one.
The problem
Two people solve the same number of problems. Who won?
How CodeDuel handles it
ICPC-style scoring ranks by problems solved, then by penalty time. Every tie has an answer.
The problem
The scoreboard gives away the last stretch, so everyone plays to it instead of the problems.
How CodeDuel handles it
A freeze window before the end hides the final standings until the contest is over.
The problem
A winner is announced while submissions are still waiting to be judged.
How CodeDuel handles it
A contest is only finalised once every submission has a verdict.
The problem
Someone is late, or cannot make it after all.
How CodeDuel handles it
Participants register and withdraw, and the organiser can allow late joining up to a cutoff.
The problem
Someone is accused of breaking the rules.
How CodeDuel handles it
Only contest staff can disqualify a participant, and it cannot be undone by anyone else.
Mentor
A plan you can follow, and an AI that asks first.
Solvesstudy plans that fail quietly
Most study plans fail quietly. The mentor keeps yours realistic and never changes it behind your back.
The problem
You solve problems at random and never know what to practise next.
How CodeDuel handles it
The mentor builds a week-by-week plan from the concept map, skipping what you already solved.
The problem
AI tools rewrite your work before you have agreed to anything.
How CodeDuel handles it
The mentor only proposes. Nothing is saved until you confirm, and an unanswered proposal expires after 15 minutes.
The problem
A busy week wrecks the plan, so you abandon it.
How CodeDuel handles it
When you change it, finished and past items stay fixed and only the upcoming work is rearranged.
FAQ
Questions people ask first.
What is CodeDuel?
A private competitive programming platform. You practise problems daily, every submission is judged in a sandbox, you can join contests, and an AI mentor helps plan what to solve next.
Who is it for?
People who want to practise consistently and compete with friends, classmates or study partners. There is no global leaderboard, and a private contest can be reached only through its invitation link.
Which languages can I use, and do I install anything?
Python 3.12 and Java 21. You write and run code in the browser, so there is nothing to install.
Is it safe to run code here?
Yes. Each run gets its own container with no network, hard CPU and memory limits and a read-only filesystem, and the container is destroyed afterwards.
Will the AI change my plan without asking?
No. The mentor only proposes changes. Nothing is saved until you confirm, and an unanswered proposal expires after 15 minutes.
Are there ratings or live 1v1 duels?
Not yet. Today you get timed ICPC-style contests with standings that freeze before the end. Ratings and live head-to-head duels are planned, not built.
Stop practising in the dark. Start with direction.
Create an account, set a goal, solve something today and see exactly how you did. It takes a minute.