Remove duplicates without using LINQ Distinct()?

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)
{
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.”