First repeating element in array?

Short answer: int[] arr = { 3, 5, 1, 5, 2 }; HashSet<int> set = new HashSet<int>(); foreach (int num in arr) { if (!set.Add(num)) { Console.WriteLine(num); break; } }

Example code

int[] arr = { 3, 5, 1, 5, 2 };
HashSet<int> set = new HashSet<int>();
foreach (int num in arr)
{
if (!set.Add(num))
{ Console.WriteLine(num); break; }
}

Real-world example (ShopNest)

In coding rounds, state complexity aloud, write a clear ShopNest-flavored example (orders, carts), then handle edge cases (empty list, null, overflow).

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.