03 — Problem Banks¶

Problem banks are where learning becomes ability. Books and videos build the mental model; problem banks reveal whether that model is actually working under pressure. The test is not “do I understand this algorithm” — it’s “can I recognize when to use it and implement it correctly in 20-40 minutes with no hints.”

The mistake most learners make is staying on one platform — usually LeetCode — indefinitely. LeetCode is an excellent interview prep tool but a poor CP development tool. Codeforces builds different muscles: time pressure, unfamiliar problem framing, adversarial test cases. Both are required for this journey’s goals.

1. LeetCode¶

Structure: 3000+ problems organized by Easy/Medium/Hard. Topic tags (arrays, DP, trees, etc.). Company tags (Premium). Discussion forum per problem. Weekly and biweekly timed contests.

How to use it effectively (not just grind): LeetCode grinding without structure produces pattern blindness — you solve 200 problems but can’t categorize a new one. The correct approach:

Follow the NeetCode 150 list in order. This is not “solve random problems” — it’s a curated set that covers every pattern that appears in FAANG interviews.
Per problem: attempt 20-30 minutes, then watch the NeetCode video, then implement from scratch without looking. The “implement from scratch after explanation” step is what most people skip and why they don’t improve.
After solving: look at the Discuss tab. Read the top-voted alternative solution. It is almost always a different approach or a cleaner implementation that teaches something.
Use topic-tag filtering once you’ve done NeetCode 150 — drill weak areas.

Premium vs. free: Free tier covers all 3000+ problems except ~400 Premium-locked ones. NeetCode 150 is entirely free. The premium value is company-specific tags — skip until Phase 5.

Blind 75 vs. NeetCode 150: NeetCode 150 is the current standard. Blind 75 is the predecessor — still useful but subsumed by NeetCode 150, which covers all Blind 75 problems plus important additions. Start with NeetCode 150.

Phase alignment: Phases 1-6. Heaviest use in Phases 1-4.

Cost: Free (Premium: $159/yr, defer to Phase 5).

Verdict for this learner: PRIMARY problem bank for LeetCode track throughout. The NeetCode 150 list is your Phase 1-2 problem schedule. After completing it, filter by difficulty and topic for deep practice.

URL: leetcode.com

2. Codeforces¶

Structure: Active competitive programming platform. Multiple contest divisions: Div. 1 (expert+), Div. 2 (1400-2100 rating), Div. 3 (beginner-1600 rating), Div. 4 (absolute beginner). Problems rated by difficulty (800 to 3500+). Virtual contests (simulate past contests). Problem tags.

How to use as a beginner (start Div. 3, not Div. 2): The most common mistake: beginners join a Div. 2 contest, get two problems out of five, feel destroyed, and conclude CP is not for them. The correct approach:

Phase 3 start: Register and solve Div. 3 problems rated 800-1200. These are solvable with solid foundations. They are the calibration tool.
Virtual contests: Use the “Virtual Contest” feature to simulate past Div. 3 and Div. 4 contests with a real timer. This is the single best simulation of the actual CP experience without the stakes.
Problem tags: After virtual contests, review problems you didn’t solve. Use the problem tag to identify the concept, study it, then reattempt.
Rating system: Your rating starts at 0 and adjusts after rated contests. Div. 3 puts beginners in a fair environment. Target: reach 1200 (Pupil) by end of Phase 4, 1600 (Specialist) by end of Phase 5.
Blog section: The Codeforces blog has editorial explanations for every past contest. These editorials are often the clearest explanations of an algorithm in context.

Phase alignment: Phase 3 onwards. Occasional virtual contests from Phase 2 for exposure.

Cost: Free.

Activity level (2025-2026): Active. Codeforces Round #968 (Div. 3) was running in 2026. Weekly contests. Rating is globally recognized.

Verdict for this learner: PRIMARY CP platform, Phase 3+. LeetCode builds interview skills; Codeforces builds competitive programming skills. Both are required for this journey’s goals.

URL: codeforces.com

3. AtCoder¶

Structure: Japanese competitive programming platform. Primarily known for AtCoder Beginner Contests (ABC). Weekly Saturday/Sunday contests. 26-30 problems per ABC, rated A-F by difficulty. ABC covers up to rating ~1999 (Div. 2 equivalent on Codeforces). Problems are known for mathematical elegance and clean problem statements.

How to access from India: Full access — no geo-restrictions. AtCoder account creation is straightforward. Contests run at 21:00 JST = 17:30 IST (Sunday afternoon, accessible for Indian schedules).

Community verdict on problem quality: r/competitiveprogramming (2024-2026) consistently praises AtCoder ABC problems for clarity and mathematical precision. “AtCoder problem statements are the cleanest in CP — no ambiguous edge cases, no poorly translated problems.” Especially recommended for learning DP patterns — many landmark DP problems originated or were popularized through AtCoder contests.

Phase alignment: Phase 3-6. ABC A-B problems from Phase 2 for exposure. ABC C-D problems from Phase 3 for core CP practice.

Cost: Free.

Activity level: Weekly. ABC 468+ was announced in July 2026 — platform is clearly active.

Verdict for this learner: SUPPLEMENT to Codeforces for CP practice, Phase 3+. AtCoder and Codeforces cover different problem styles. AtCoder is particularly strong for DP and combinatorics problems. Participate in at least one ABC per month from Phase 3 onwards.

URL: atcoder.jp

4. CSES Problem Set¶

Structure: Curated set of ~400 competitive programming problems (reached 400 in May 2025 per cses.fi/blog). Problems organized by topic: sorting and searching, dynamic programming, graph algorithms, tree algorithms, mathematics, string algorithms, advanced techniques. No timed contests — pure problem bank.

Why it’s exceptional: CSES is not a random problem collection. It is a carefully chosen set where each problem tests a specific concept at a well-calibrated difficulty. Solving the CSES DP section will give you stronger DP fundamentals than random LeetCode grinding. Princeton CP team, USACO Guide, and TLE Eliminators (India) all reference CSES as a primary structured practice resource.

Community verdict: r/competitiveprogramming (2024-2026): “CSES is the best structured problem set that exists for CP fundamentals. Every problem teaches something specific.” TLE Eliminators (India-based CP community, active 2025-2026) produces video solutions for CSES problems — making it accessible even when you’re stuck. The CSES editorial section (partly maintained) and CPH (Competitive Programmer’s Handbook) by the same author (Laaksonen) serve as the canonical reference.

Phase alignment: Phases 2-5. Start with the sorting and searching section in Phase 2. DP section in Phase 3. Graph section in Phase 3-4. Advanced techniques in Phase 4-5.

Cost: Free.

Verdict for this learner: PRIMARY structured CP problem bank, Phases 2-5. Use it as the primary problem source when working through CP Handbook topics. The topic alignment between CP Handbook and CSES is intentional — they’re by the same author (Laaksonen).

URL: cses.fi/problemset (verify before use)

5. HackerRank¶

Structure: Problems organized by topic and difficulty. Company-hosted coding tests. HackerRank certification badges.

Current community verdict (2025-2026): r/learnprogramming and r/cscareerquestions: “HackerRank is useful if your target company specifically uses their testing platform for screening. Otherwise stick with LeetCode.” The platform is heavily used by companies for automated screening (especially in India — many MNC/service company recruitment funnels run through HackerRank). The problem quality and educational value are lower than LeetCode or CSES.

India-specific note: If you’re targeting companies that use HackerRank for their initial screening (common for service companies, some MNCs), practice on their platform specifically. The interface and time pressure are different from LeetCode. If your targets are product companies or FAANG-style interviews, HackerRank is not the training ground.

Phase alignment: Late Phase 5-6 only, and only if specifically targeting companies that use HackerRank screening.

Cost: Free (premium certifications are irrelevant for this journey).

Verdict for this learner: LOW PRIORITY. Use only if a target company uses it for screening. For DSA skill building, it’s strictly inferior to LeetCode + CSES + Codeforces.

6. CodeChef¶

Structure: Indian CP platform. Contests: Long Challenge (10 days), Cook-Off (2.5 hours), Starters (2 hours). Problems organized by difficulty and topic.

History and controversy: Founded 2009 by Directi. Acquired by Unacademy in June 2020. Hived off as independent March 2023 (Unacademy retains 30% stake). The acquisition period (2020-2023) saw significant quality degradation: Snackdown (flagship international contest) was discontinued, community complaints about plagiarism and weak test cases accumulated (Codeforces blog entry #126245 documents community concerns). The 2023 independence has partially restored trust, but contest quality concerns persist in community discussion as of 2024-2026.

Community verdict (2024-2026): r/competitiveprogramming and Codeforces blogs: mixed. The positive view: CodeChef still hosts accessible contests with a large Indian participant base; the Long Challenge format (10 days) is a unique format unavailable elsewhere. The negative view: test case quality and plagiarism enforcement remain concerns; Codeforces is strictly better for serious CP development.

India-specific value: The ICPC regional contests in India previously ran through CodeChef infrastructure. With Snackdown discontinued, this pipeline has weakened. However, CodeChef Starters (2-hour contests, every Wednesday 8pm IST) remain accessible for Indian schedules.

Phase alignment: Phase 3-5, specifically for ICPC regionals if that’s a target. Starters are worth participating in occasionally.

Verdict for this learner: SUPPLEMENT, not primary. Use Codeforces and AtCoder as primary CP platforms. Participate in CodeChef Starters for the India-accessible schedule and ICPC regional exposure.

URL: codechef.com

7. Kattis¶

Structure: CP problem judge used extensively by North American and European universities. ~5000+ problems. Many ICPC-style problems. Problems sourced from real regional/world contests.

Community verdict: r/competitiveprogramming: Kattis is primarily for university CP teams doing ICPC preparation. The problem quality is high (real contest problems) but the platform UI and organization are inferior to CSES/Codeforces for self-directed learning. It is not commonly recommended for self-study outside of ICPC team contexts.

Phase alignment: Phase 5-6 only, and only if targeting ICPC specifically.

Cost: Free.

Verdict for this learner: SKIP unless targeting ICPC. Codeforces + AtCoder + CSES cover your CP development needs. Kattis adds no marginal value over these three for a self-directed learner.

Summary Table¶

Platform	Best Use	Phase	Free/Paid	Priority
LeetCode	Interview prep, NeetCode 150	1-6	Free (Premium Phase 5+)	PRIMARY interview track
CSES Problem Set	Structured CP fundamentals	2-5	Free	PRIMARY CP problem bank
Codeforces	CP contests, rated problems	3-6	Free	PRIMARY CP platform
AtCoder	Clean problems, DP/math focus	3-6	Free	SUPPLEMENT to Codeforces
CodeChef	India schedule, ICPC context	3-5	Free	SUPPLEMENT (India)
HackerRank	Company screening only	5-6	Free	LOW PRIORITY
Kattis	ICPC preparation	5-6	Free	SKIP unless ICPC

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