Combinatorics is the branch of mathematics that focuses on counting, arranging, and selecting objects. In coding interviews, it often appears in problems involving permutations, combinations, subsets, and ways to distribute or arrange elements under certain constraints. Many interview questions require you to efficiently count possibilities without explicitly generating every outcome.
Combinatorics problems frequently appear alongside algorithmic techniques such as Dynamic Programming, where you count valid states, or Backtracking and Recursion, where you explore combinations of elements. A strong understanding of mathematical principles from Math also helps when working with factorials, binomial coefficients, and modular arithmetic.
Common patterns you’ll encounter include:
Mastering combinatorics improves your ability to reason about large search spaces and optimize counting problems. With 43 practice problems on TalentD DSA Corner, you can build the intuition needed to quickly recognize patterns and solve interview-level combinatorial challenges.
Provides the foundation for factorials, combinations, permutations, and modular arithmetic used in combinatorics problems.
Helps generate combinations, subsets, and permutations when exploring solution spaces.
Useful for representing subsets and iterating through combinations efficiently using bitmasks.
Many combinatorics problems count the number of valid states or sequences using DP transitions.
| Status | Title | Video | Leetcode | Solve | Difficulty | Companies | Topics |
|---|---|---|---|---|---|---|---|
| 62. Unique Paths | Solve | Medium | Accoloite+41 | ||||
| 1201. Ugly Number III | Solve | Medium | American Express | ||||
| 1621. Number of Sets of K Non-Overlapping Line Segments | Solve | Medium | Amazon | ||||
| 1641. Count Sorted Vowel Strings | Solve | Medium | Google+1 | ||||
| 2063. Vowels of All Substrings | Solve | Medium | ServiceNow | ||||
| 2221. Find Triangular Sum of an Array | Solve | Medium | Amazon | ||||
| 2400. Number of Ways to Reach a Position After Exactly k Steps | Solve | Medium | Amazon | ||||
| 2539. Count the Number of Good Subsequences | Solve | Medium | Oracle+1 | ||||
| 2597. The Number of Beautiful Subsets | Solve | Medium | Facebook+1 | ||||
| 2638. Count the Number of K-Free Subsets | Solve | Medium | Amazon | ||||
| 2929. Distribute Candies Among Children II | Solve | Medium | Amazon+1 | ||||
| 2930. Number of Strings Which Can Be Rearranged to Contain Substring | Solve | Medium | Meesho | ||||
| 3128. Right Triangles | Solve | Medium | - | ||||
| 3179. Find the N-th Value After K Seconds | Solve | Medium | - | ||||
| 3247. Number of Subsequences with Odd Sum | Solve | Medium | - |
Start Easy, progress to Hard.
Frequently appear alongside Combinatorics.
Common questions about Combinatorics.
It sits at the intersection of both. You use mathematical counting principles together with algorithmic techniques to compute results efficiently for large inputs.
Yes, especially in companies that test problem-solving and mathematical reasoning. They often appear in problems involving counting arrangements, subsets, or ways to form sequences under constraints.
Combinatorics in DSA focuses on counting and arranging elements efficiently without enumerating every possibility. It commonly involves permutations, combinations, and subset counting techniques.
Practicing around 30–50 well‑selected problems is usually enough to understand common patterns. TalentD’s set of 43 Combinatorics problems is designed to cover the most common interview scenarios.
Typical approaches include combinatorial formulas, recursion/backtracking, dynamic programming for counting states, and bitmasking for subset enumeration.