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
two-sum · Python 3.12
VerdictAccepted
- Sample 1
- Sample 2
- Hidden 1
- Hidden 2
- Hidden 3
Practice
A focused place to get better, every day.
Less browsing, more solving. CodeDuel organises problems around the ideas behind them.
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
Hostile code, safely contained.
Submitted code is treated as untrusted. Every run happens in its own locked-down container, and the result is a precise verdict, 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.
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.
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.
Your next problem is waiting.
Create an account, solve something today, and see exactly how you did.