🔢 Math

Math problems reward recognizing the structure: modular arithmetic, GCD, primality, combinatorics, or simple geometry. The right formula usually beats any data-structure cleverness.

This category contains 25 problems. Use the patterns below to recognize what's being asked, then jump to the problem list at the bottom.

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🧠 Key Patterns

• Modular Arithmetic — (a + b) % m, modular inverse via Fermat.
• GCD / LCM — Euclidean algorithm. lcm = a / gcd * b.
• Prime Sieve — Sieve of Eratosthenes for primes up to $n$ in $O(n \log \log n)$.
• Fast Exponentiation — Power in $O(\log n)$ via repeated squaring.
• Combinatorics — $nC_r$, Pascal's triangle, stars and bars.
• Geometry — Cross product for orientation, distance formula, line intersection.

⚠️ Common Pitfalls

• Integer overflow on multiplication — promote to long early.
• Negative modulo: (-7) % 3 is -1 in C#, not 2. Add m and re-mod.

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📚 Study Resources

📺 Videos

• 3Blue1Brown — Essence of Calculus / Number Theory
• Errichto — Math for CP

📖 Books

• Concrete Mathematics — Graham, Knuth, Patashnik — The DP-of-math book
• Competitive Programming Handbook — Laaksonen — Ch. 21

🌐 Articles & References

• CP-Algorithms — Math section

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💻 All Math Problems

📄️Add Digits

LeetCode 258 | Difficulty: Easy

📄️Add Two Integers

LeetCode 2383 | Difficulty: Easy

📄️Angle Between Hands of a Clock

LeetCode 1446 | Difficulty: Medium

📄️Bulb Switcher

LeetCode 319 | Difficulty: Medium

📄️Complement of Base 10 Integer

LeetCode 1054 | Difficulty: Easy

📄️Copy List with Random Pointer

LeetCode 138 | Difficulty: Medium

📄️Count Integers With Even Digit Sum

LeetCode 2298 | Difficulty: Easy

📄️Count Primes

LeetCode 204 | Difficulty: Medium

📄️Fibonacci Number

LeetCode 1013 | Difficulty: Easy

📄️Find N Unique Integers Sum up to Zero

LeetCode 1426 | Difficulty: Easy

📄️Fizz Buzz

LeetCode 412 | Difficulty: Easy

📄️Happy Number

LeetCode 202 | Difficulty: Easy

📄️Integer to Roman

LeetCode 12 | Difficulty: Medium

📄️Largest Rectangle in Histogram

LeetCode 84 | Difficulty: Hard

📄️Minimum Area Rectangle

LeetCode 976 | Difficulty: Medium

📄️Pow(x, n)

LeetCode 50 | Difficulty: Medium

📄️Rectangle Area

LeetCode 223 | Difficulty: Medium

📄️Rectangle Overlap

LeetCode 866 | Difficulty: Easy

📄️Reverse Integer

LeetCode 7 | Difficulty: Medium

📄️Roman to Integer

LeetCode 13 | Difficulty: Easy

📄️Shuffle an Array

LeetCode 384 | Difficulty: Medium

📄️Sqrt(x)

LeetCode 69 | Difficulty: Easy

📄️Sum of Square Numbers

LeetCode 633 | Difficulty: Medium

📄️Three Divisors

LeetCode 2083 | Difficulty: Easy

📄️Ugly Number

LeetCode 263 | Difficulty: Easy