Arrays & Hashing

Overview

Arrays & Hashing is the first section of the NeetCode 150 and the most load-bearing, a hash map + array reasoning pattern appears in most of the remaining categories. Master these nine and you’ve unlocked roughly 30% of the full list’s problem-solving surface.

Problems

1. 217. Contains Duplicate (Easy)
2. 242. Valid Anagram (Easy)
3. 1. Two Sum (Easy)
4. 49. Group Anagrams (Medium)
5. 347. Top K Frequent Elements (Medium)
6. 238. Product of Array Except Self (Medium)
7. 36. Valid Sudoku (Medium)
8. 271. Encode and Decode Strings (Medium) (LC premium, also appears as #659)
9. 128. Longest Consecutive Sequence (Medium)

Key patterns unlocked here

• Hash set for dedup / membership, Contains Duplicate, Longest Consecutive Sequence.
• Frequency counting, Valid Anagram, Group Anagrams, Top K Frequent Elements.
• Complement lookup (one-pass hash), Two Sum, and dozens of variants downstream.
• Prefix / suffix products or sums, Product of Array Except Self.
• Encoding state into a hashable key, Group Anagrams (char-count tuple), Valid Sudoku (row/col/box keys).