AtCoder Beginner’s Guide [2025] The Fastest Route to Brown Coder & Essential Algorithms

Introduction

AtCoder is Japan’s largest competitive programming contest site. As of 2026, it has over 600,000 registered users, with more than 10,000 participants joining the contests held every Saturday at 9 PM. This article introduces effective strategies for those who want to start AtCoder or improve their rating.

AtCoder’s Rating System

AtCoder assigns ratings based on contest performance. Specifically, your rating determines your color: starting from gray, then progressing through brown, green, cyan, blue, yellow, orange, and red. Reaching green is considered quite excellent as an engineer. Let’s first aim for brown coder status.

5 Steps to Becoming a Brown Coder

To reach brown coder (rating 400), you need to reliably solve A and B problems in AtCoder Beginner Contest (ABC), and be able to solve easier C problems. Follow these 5 steps for efficient learning.

Step 1: Learn Programming Basics

We recommend solving the “Introduction To Programming I” course. Completing all 40 problems will teach you the programming fundamentals needed for competitive programming (input/output, conditionals, loops, arrays, etc.). C++ or Python are the recommended languages.

Step 2: Understand Computational Complexity and Algorithms

In competitive programming, you need not only correct answers but also solutions that finish within the time limit. As a rule of thumb, you can run about 100 million loops per second. Understanding computational complexity (Big-O notation) lets you judge whether your program will finish within the time limit.

Step 3: Solve AtCoder Beginners Selection

Solve the 11 problems in AtCoder’s official beginner problem set, “AtCoder Beginners Selection.” This helps you grasp AtCoder’s problem patterns and basic solution approaches. Detailed editorial articles are available, so read them for problems you can’t solve.

Step 4: Master Brute Force Search

The most important algorithm for becoming a brown coder is “brute force search” (exhaustive search). This means checking every possible pattern. Types include nested loops, bit-based exhaustive search, and permutation-based exhaustive search. Over 70% of ABC B problems can be solved with brute force or simple implementation.

Step 5: Keep Participating in Contests

The most important thing is to keep participating in the weekly contests. Data shows that people who participate more have higher median ratings: 241 after 5 contests, 521 after 10, and 884 after 20. Don’t worry about failures — consistency is the fastest route to improvement.

Recommended Learning Resources

• AtCoder Problems: The best site for searching and managing past problems. View all your solved problems at a glance at kenkoooo.com
• APG4b: AtCoder’s official C++ tutorial material. Even programming beginners can learn the knowledge needed for competitive programming
• Programming Contest Challenge Book (“Ant Book”): Considered the bible of competitive programming. Systematically teaches algorithms
• Algorithms x Mathematics Book: Learn the mathematical knowledge and algorithms needed for competitive programming

Essential Algorithms for Brown Coders

To aim for brown (rating 400+) on AtCoder, master the following algorithms.

Brute Force Search

The most basic technique of checking all possibilities. Brute force problems frequently appear in ABC A and B problems. If computational complexity allows, always consider brute force first.

Sorting and Binary Search

Techniques for sorting data and searching efficiently. Frequently appears in C problems and beyond, reducing complexity to O(N log N). In C++, master the sort function and lower_bound; in Python, master sorted and the bisect module.

Prefix Sums

A technique for quickly computing range sums. O(N) preprocessing, then O(1) per query for range sums. When you see “sum of a range” in ABC C or D problems, suspect prefix sums.

Greedy Algorithms

A method of repeatedly making the locally optimal choice. The interval scheduling problem (selecting by earliest end time) is a classic example. Intuitive to understand and relatively simple to implement.

Language Choice: C++ vs Python?

The popular languages for competitive programming are C++ and Python. Understand each’s characteristics and choose what suits you.

• C++: Overwhelmingly fast execution speed, making it easier to pass problems with tight time limits. Almost all top AtCoder competitors use C++. However, the syntax is somewhat complex and challenging for beginners
• Python: Code is concise and readable, making it approachable for beginners. However, execution speed is slower than C++, sometimes hitting time limits on D problems and beyond. Using PyPy (fast Python) can improve this somewhat
• Recommendation: Programming beginners should start with Python to build fundamentals, then consider switching to C++ after reaching brown. This is a well-balanced strategy

Efficient Contest Participation Strategy

• Participate in ABC every week: ABCs are held every Saturday at 9 PM. To aim for brown, build the habit of reliably solving A and B problems while attempting C problems
• Always review unsolved problems: Read official editorials after contests and compare with your approach. This “editorial AC” review is the biggest shortcut to improvement
• Use AtCoder Problems: Filter past problems by difficulty on the community-run AtCoder Problems site. Filling in gray difficulty (0-399) problems is the fastest route to brown
• Practice with virtual contests: Use the “virtual contest” feature to solve past contests under the same time constraints. This helps develop time management skills

Why Competitive Programming Helps with Job Hunting

AtCoder ratings are increasingly noticed in IT company hiring. AtCoder’s “AtCoder Jobs” offers a rating-based job matching service. Being brown coder or above proves you have fundamental algorithm knowledge. Major IT companies like Yahoo Japan have set AtCoder ratings as application requirements for engineer positions. As more companies adopt coding tests, competitive programming experience provides a significant advantage.

Frequently Asked Questions (FAQ)

Q. What age can you start participating in AtCoder?

There are no age restrictions. Participants range from elementary school students to working adults. Some middle school students have even reached cyan or blue coder status.

Q. How long does it take to become a brown coder?

It varies by individual, but starting from zero programming experience, studying 1-2 hours daily can often lead to brown in about 3-6 months. Those with existing programming experience can potentially reach it in 1-2 months.

Q. Does AtCoder cost anything to participate?

Registration and contest participation on AtCoder are completely free. While there are paid AtCoder Library books available, free online materials are more than sufficient for learning.

Common Stumbling Blocks and Solutions

Can’t Estimate Computational Complexity

Understanding computational complexity (Big-O notation) is where beginners struggle most. AtCoder’s time limit is typically 2 seconds — roughly 10^8 operations for C++ and 10^6 for Python. Build the habit of checking input size N first, then judging whether O(N^2) will pass or if O(N log N) is needed.

Dealing with WA and TLE

When you get WA (Wrong Answer), first check corner cases — test with N=0, N=1, maximum and minimum values. When you get TLE (Time Limit Exceeded), you need to reconsider your algorithm. Check whether you can replace brute force with sorting or binary search.

Maintaining Motivation

Everyone goes through periods where their rating doesn’t improve as expected. When that happens, look back at your rating history on your AtCoder user page. Over the long term, it should definitely be trending upward. Finding competitive programming friends on social media for information exchange is also effective.

What to Aim for After Brown Coder

After reaching brown, aim for green (rating 800+). Additional skills needed for green include dynamic programming (DP), graph traversal (BFS/DFS), and Union-Find. Dynamic programming is especially the most important algorithm, frequently appearing in ABC D and E problems. AtCoder has a permanent “Educational DP Contest” — tackle it after reaching brown. Reaching green earns very high evaluations in IT company hiring.

Things to Know Before Starting Competitive Programming

• Minimal setup is fine: AtCoder’s online editor is sufficient at first. Local environment setup can wait until you’re comfortable. C++ code can be run directly with AtCoder’s code test feature
• High school math is enough: Advanced math isn’t needed for brown. Basic arithmetic, modular arithmetic, and logical thinking with conditionals are sufficient. Don’t worry if math isn’t your strong suit
• Don’t compare yourself too much: Competitive programming is a battle with yourself. While social media shows people with incredible rating growth, the vast majority build up steadily. Progress at your own pace
• Having fun is the top priority: Whether you can enjoy the puzzle-solving aspect determines whether you’ll continue long-term. Approaching it from intellectual curiosity rather than obligation leads to much faster improvement

Besides AtCoder, there are other competitive programming platforms. Codeforces is the world’s largest competitive programming site — it’s in English but offers high-quality problems and international ratings. LeetCode specializes in coding interview preparation and is popular with engineers in job transitions. However, for Japanese language support and beginner-friendliness, AtCoder is by far the best choice. Build your foundation on AtCoder first, then try other platforms.

Conclusion: Just Enter This Weekend’s ABC

The path to becoming a brown coder on AtCoder is by no means steep. Master basic algorithms like brute force and sorting, participate in weekly ABCs, and consistently review — your rating will steadily climb.

The only two things that matter are “keep participating in contests every week” and “always review problems you couldn’t solve.” Enter this weekend’s ABC first. It’s totally fine if you can’t solve anything at first. Keep going, and you’ll definitely feel yourself growing. The world of competitive programming is deep and full of friends.

ブラウザだけでできる本格的なAI画像生成【ConoHa AI Canvas】

swiftwand

X

AIを使って、毎日の生活をもっと快適にするアイデアや将来像を発信しています。 初心者にもわかりやすく、すぐに取り入れられる実践的な情報をお届けします。 Sharing ideas and visions for a better daily life with AI. Practical tips that anyone can start using right away.