Remove duplicates without using LINQ Distinct()?

Short answer: Remove duplicates without using LINQ Distinct() What interviewers test Hashing fundamentals Time vs memory trade-offs Whether you understand why Distinct() works Real-world scenario You are processing millions of user IDs coming from logs or Kafka messages. You must remove duplicates fast and predictably. Optimized approach (HashSet) public static List<int> RemoveDuplicates(int[] input)

Example code

{
var seen = new HashSet<int>();
var result = new List<int>();
foreach (var item in input)
{
if (seen.Add(item)) // Add returns false if already exists
{ result.Add(item); }
}
return result;
} Complexity Aspect Value Time O(n) Space O(n) (hash storage) Why this is better than naive loops Nested loops = O(n²) ❌ HashSet = constant-time lookup ✅ Interview Tip: Say “I prefer HashSet because it gives O(1) average lookup and preserves intent clearly.”

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.