FAQ
The most common questions, grouped by role. Each answer is short and links to the page with the full details. For the meaning of any term, see the Glossary.
Students and learners
Why do I get Wrong Answer (WA) when my output looks exactly right?
The usual culprit is extra text such as Enter n: or The answer is:. The default checker ignores extra spaces but not extra words, and line breaks between numbers must match. Also check for integer overflow (int only holds up to about 2·10⁹) and for reading or writing files when the problem uses standard input and output. See Troubleshooting.
Which languages does LCOJ support, and which compiler versions?
Open /runtimes/ on the site to see the languages that currently have a judge, with their real versions. The language dropdown on the submit page shows versions too. For C++ pick C++17 or C++20; for heavy Python problems, try PyPy 3. See Supported languages.
Can I see a problem's test cases?
By default only the problem's authors can. A setter can make tests visible to everyone, or visible only when you're not in a contest; the submission page then shows the input, the expected answer and your output for each test. See Per-test results.
Do I lose points for submitting many times?
Not in practice: the system keeps only your best score for each problem. In contests, penalties depend on the contest format, and some contests cap the number of submissions per problem.
Why is my submission stuck on "Queued"?
It's waiting for a free judge that supports your language and has the problem's data. Each user can have at most 2 submissions queued or being graded at once. If it waits too long, tell an admin. See Submitting and judging.
Can I read other people's code?
By default you can read other people's code only for problems you have solved. Setters can change this per problem. See Viewing other people's solutions.
How does rating work?
Your rating only changes after contests marked as rated, once the contest has ended and an admin runs the rating calculation. LCOJ uses the Elo-MMR algorithm (inherited from DMOJ); virtual participations don't count. The color tiers range from Newbie (below 1200) to Legendary Grandmaster (2900 and up). See Taking part in contests.
I can't remember how I signed in, or I forgot my password. What now?
On luyencode.net, accounts are created through Google, so just sign in with Google using the same Google account. The Quên mật khẩu? (Forgot your password?) link only applies to password accounts (for example, accounts a teacher bulk-created for a class) and needs the site to send email. luyencode.net doesn't have email sending configured, so ask an admin to reset the password in the admin site (Managing users). See Account and sign-in.
I lost the phone with my two-factor authentication app. What now?
When you turned on two-factor authentication, you were given a list of one-time 16-character scratch codes. Enter one of them in the 6-digit code field to sign in, then turn 2FA off or set it up again. If you have no scratch codes left, only an admin with the right permission can turn off 2FA for you. See Account and sign-in.
Problem setters and teachers
How do I create a class on LCOJ?
A class is an organization. Creating one at /organizations/create requires the judge.add_organization permission, and by default each user can administer at most 3 organizations. On luyencode.net, contact the LCOJ team if you don't have the permission yet. See Organizations (groups, classes).
How do I make problems that only my class can see?
Create the problem inside the organization (from the organization's page; this needs the judge.create_organization_problem permission). An organization's private problems are visible only to its members. Contests and quizzes can also be restricted to an organization. See Organizations (groups, classes).
Why can't anyone see the problem I just created?
New problems stay private until someone with the right permission makes them public. That gives you time to prepare tests and test-submit before anyone sees it. See Managing problems.
Can I import existing problems?
Yes, if the problem has a Codeforces Polygon package: import it at /problems/import-polygon (requires judge.import_polygon_package). The importer creates the statement, tests and checker. For anything else, create the problem on the site and upload the tests as a zip file. See Managing problems.
Can I bulk-import quiz questions?
Yes. Open /quizzes/import/, upload an Excel (.xlsx) or JSON file, check the preview and confirm. A single invalid question means nothing is imported, so fix every error shown in the preview. See Creating quizzes.
Operators and self-hosters
What server do I need?
At minimum 2 CPU cores, 4 GB RAM, 20 GB free disk and 64-bit Linux (Ubuntu 22.04 or newer is easiest); 4 cores, 8 GB RAM and a 50 GB SSD are recommended. Each judge uses roughly one extra CPU core while grading. See Installing with Docker.
Is the judge included in Docker Compose?
No. Docker Compose runs the website and bridged only; judges run separately (usually from the vnoj/judge-tier3 image) and connect on port 9999. See Setting up judges.
How do I add judges to grade faster?
Each judge grades one submission at a time. To add one, register another judge on the website with its own name and key, create its own config file, and start another container. Keep the number of judges below your CPU core count, leaving some cores for the website. See Running multiple judges.
What do I need to back up?
Four things: the database (dumped with mariadb-dump), the problems/ folder (test data), the media/ folder (uploads) and your config files. Copy the backups to another machine. See Backups.
How do I update LCOJ?
Back up the database first, then update both repositories (lcoj-docker and the dmoj/repo submodule), run migrations and restart the affected services. The exact steps depend on what changed. See Updating.
Is HTTPS required?
Not for a local trial: http://localhost:8071/ is enough. For real users you should have it. nginx inside Docker only serves HTTP, so put a TLS reverse proxy in front of it on your server, such as Caddy or nginx with certbot. See HTTPS.
Can I rebrand LCOJ with my school's name and logo?
Yes. The site name lives in SITE_NAME and SITE_LONG_NAME in local_settings.py; the logo is set with the site_logo configuration item in the admin panel. LCOJ is AGPL-3.0, so if you modify the code and let others use it over a network, you must publish your modified source. See Environment variables, Site configuration and content and License.
Support and contact
Where do I report a mistake in a statement or its tests?
Use the Báo cáo vấn đề (Report an issue) feature on the problem page to open a ticket for the setters and admins. Useful tickets also earn contribution points. See Blog, comments and tickets.
Where do I report a bug in LCOJ itself?
Open an issue on lcoj-docker's GitHub Issues. Judge problems (a language misbehaving, sandbox errors) go to luyencode/judge-server.
I need help installing LCOJ.
LCOJ offers free installation help. Get in touch through behitek.com or luyencode.net/about/#lien-he.