Hello Codeforces!
I am glad to invite you to InfOJ April Fools Day Contest 2025 (IAFDC for short) which will start on April 5th 2025, 19:00 UTC+8. The problem setters are me and gyh20.
The contest will last for 2.5 hours with 10 tasks for you to solve. All problems are worth 100 points, and the contest will be in IOI mode with standings. You can see the past problems here.
Why is April Fool's Contest not on April Fools Day?
From the year 2023, the contest has always been held on April 5th, to match the Tomb Sweeping Day's break in Chinese schools.
Language of Problem Statements
We will provide English statements for all problems. However, in the past several years, only Chinese statements are available. You can use any LLM tool to translate them into English. Since the statements are almost always very short, the LLMs have a high chance of not making any mistakes.
Highlights of this year's IAFDC
This year, we are planning to introduce problems that your submitted code can call external LLM API-s. For example, there will be a problem that, you can write the following code:
#include "llm.h"
int main(){
string str = "Who are you?"
string res = ask_LLM(str); // res may be equal to "Greetings! I'm DeepSeek-V3, an artificial intelligence assistant created by DeepSeek. I'm at your service and would be delighted to assist you with any inquiries or tasks you may have."
}
However, be careful enough when using such functions, because the return value and the time taken to return the answer are unpredictable.
What are the problems of IAFDC like?
Since 2021, InfOJ has held four April Fool's Contests. Each year, we uphold the mission of "expanding the boundaries of the online judging system" and present freshly eye-opening problems to a wide range of participants, both with and without competitive programming background. We are committed to leveraging the versatility of the open-source UOJ System (which InfOJ is based on) to provide testing for problems that cannot be judged on other online judging systems. Here are some examples of such problems from past years:
- Problem A from IAFDC 2024. Statement: you need to print an integer in $$$[0,2\times 10^9]$$$.
- Problem A from IAFDC 2023. Statement: Print your birthday.
- Problem F from IAFDC 2024. Statement: Submit the yellow chicken. (There is an image of a yellow chicken in the statements)
- Problem J from IAFDC 2024. Statement: Print an integer in $$$[0,10^9]$$$.
- Problem G from IAFDC 2023. Statement: The test data has been sent to you (but there isn't a download link in the statements). Just write a program that outputs the corresponding output and submit!
There will also be problems that are codeforces-april-fools-like. You need to guess the problem statement and write the corresponding solution. However, we tend to make the real statement connected with either OJ platforms or interesting facts in computer science. Here are some examples from past years:
- Problem B from IAFDC 2024. Statement: Given a valid problem ID on InfOJ, print 0 or 1.
- Problem A from IAFDC 2022. Title: Infinity. Statement: print a 128-bit 01 string.
There will be no puzzle-solving or decryption problems which appears frequently in some other OJ's April Fools Contests. We are devoted to make problems as "clean" as possible, and solvable without unfriendly techniques.
To make the contest more approachable, there will be trivial problems that are aimed to make everyone have fun. Here are examples of such problems:
- Problem C from IAFDC 2024. Statement: Find a shortest hamiltonian cycle in a graph of $$$n$$$ vertices. The length of the edge between vertices $$$i$$$ and $$$j$$$ is $$$\gcd(i,j)$$$.
