Combinatorics Problems
42 Combinatorics problems on LeetCode, a Math & Geometry interview pattern, ranked by importance (how often it comes up in real interviews). Difficulty (ELO) and importance shown per problem.
| # | PROBLEM | DIFF | ELO | IMP | PATTERNS |
|---|---|---|---|---|---|
| 1 | Vowels of All Substrings | Medium | 1663 | 55 | Prefix Sums |
| 2 | Count All Valid Pickup and Delivery Options | Medium | 1723 | 53 | Linear Recurrence DP · Modular Arithmetic |
| 3 | Ways to Split Array Into Good Subarrays | Medium | 1598 | 48 | Frequency Counting / Hash Map Lookup · Modular Arithmetic |
| 4 | Number of Ways to Reorder Array to Get Same BST | Hard | 2288 | 48 | BST Operations (Insert, Delete, Validate) · Linear Recurrence DP |
| 5 | Count the Number of Arrays with K Matching Adjacent Elements | Hard | 2310 | 48 | Modular Arithmetic |
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