GPT-7 - COI 2026 Round A Problem

A competitive programming problem I designed for Round A of the Cyprus Olympiad in Informatics (COI) 2026.

About

This repository contains:

• Problem statements (English and Greek, in both .md and PDF formats)
• Solutions in C++ for all 6 subtasks
• Test cases
• Test case generation code

Problem Design

Inspiration: This problem was inspired by concepts from my linear algebra and machine learning classes during my 1st semester as an undergraduate at Rice.

Core Concept: Built around the prefix sum technique, a fundamental optimization in competitive programming.

Skills Tested:

• Algorithmic knowledge: Prefix sums, Sorting, Cumulative Sums, Range Queries
• Data structures: Multi-dimensional arrays, Tuples/Pairs
• Implementation: Subtasks 3 and 6 are relatively implementation-heavy and test the ability to write and debug larger programs.
• Problem-solving: Recognizing how to use dual prefix sums for alternating sums.

Test Case Generation

The test cases were generated using a custom C++ generator (gen.cpp) that creates random inputs based on subtask-specific constraint settings that I wrote manually. The generation process:

1. Settings files in subtask_gen_settings/ define constraints for each subtask (N, M, K ranges, special conditions)
2. Generator script (gen.cpp) reads these settings and produces random valid inputs
3. Reference solution (sub6_solution.cpp) processes each input and calculates the expected outputs
4. Automation script (run.sh) orchestrates the generation of all test cases and organizes them in files and folders.

Subtasks:

The 6-subtask structure allows contestants to earn partial credit while solving and testing different parts of the problem and applying optimization techniques progressively.

• Subtask 1, 2 (10 pts): Trivial - Test problem understanding
• Subtask 3 (25 pts): Brute Force
• Subtask 4,5,6 (65 pts): Build upon the Prefix Sum Technique

Repository Structure

├── GPT-7 (EN).md/pdf         # Problem statement (English)
├── GPT-7 (GR).md/pdf         # Problem statement (Greek)
├── editorial.md              # Solution explanations
├── solutions/                # C++ solutions for subtasks 1-6
│   ├── sub1.cpp
│   ├── ...
│   └── sub6.cpp
├── test_case_generator/      # Scripts and settings used for generating the test cases
│   ├── subtask_gen_settings/    # Generator settings for each subtask
│   ├── gen.cpp                  # C++ script that randomly generates test cases
│   ├── run.sh                   # Bash script for test case generation
│   └── sub6_solution.cpp        # Full solution used to calculate the outputs
├── test_cases/               # Test cases for subtasks 1-6
│   ├── sub1_tests/
│   ├── ...
│   └── sub6_tests/
└── images/                   # Problem Statement LaTeX & Diagrams

Competition

This problem was used in COI 2026 Round A.

• For submission and official testing, visit the COI Official Website.
• For the official editorial, visit COI Guide.

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Author: Antreas Leonidou
Date: December 2025