Find all duplicate characters in a string?

Short answer: string input = "programming"; Dictionary<char, int> map = new Dictionary<char, int>(); foreach (char c in input) map[c] = map.ContainsKey(c) ? map[c] + 1 : 1; foreach (var item in map) { if (item.Value > 1) Console.WriteLine(item.Key); }

Example code

string input = "programming";
Dictionary<char, int> map = new Dictionary<char, int>();
foreach (char c in input)
map[c] = map.ContainsKey(c) ? map[c] + 1 : 1;
foreach (var item in map)
{
if (item.Value > 1) Console.WriteLine(item.Key); }

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.