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)

Bonus problems (same pattern, outside NeetCode 150):

• 205. Isomorphic Strings (Easy)
• 228. Summary Ranges (Easy)
• 303. Range Sum Query - Immutable (Easy)
• 349. Intersection of Two Arrays (Easy)
• 387. First Unique Character in a String (Easy)
• 454. 4Sum II (Medium)
• 459. Repeated Substring Pattern (Easy)
• 1010. Pairs of Songs With Total Durations Divisible by 60 (Medium)
• 1071. Greatest Common Divisor of Strings (Easy)

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).
• Prefix sums for $O(1)$ range queries: Range Sum Query.
• Range boundaries in a sorted scan: Summary Ranges.
• Modular arithmetic for pair counting: Pairs of Songs Divisible by 60.