Hash Sets

Hash sets are one of the most fundamental data structures for coding interviews, providing O(1) membership testing and automatic uniqueness. When you need to check if an element exists, eliminate duplicates, or track what you've seen, hash sets are often the answer.

The power of hash sets comes from their simplicity and efficiency. Unlike arrays that require O(n) searching, hash sets let you check membership instantly. Unlike sorted arrays that need binary search setup, hash sets work with any hashable data. This unit covers the basics, common operations, and essential problem patterns.

What You'll Learn

• Set Basics: How hash sets work, core operations (add, remove, contains), time complexity analysis, and when to choose sets over other data structures. You'll understand the difference between sets and arrays/lists.

• Set Operations: Working with multiple sets including union, intersection, difference, and symmetric difference. Learn how these operations solve problems involving comparisons between collections.

• Set Problems: Classic interview problems that leverage sets, including detecting duplicates, finding pairs/triplets, cycle detection, and tracking visited elements in graph traversals.

Why It Matters

Hash sets appear constantly in interview problems, often as a key optimization that reduces time complexity from O(n²) to O(n). The pattern is simple: when you need to check "have I seen this before?" or "is this element present?", reach for a hash set.