First non-repeating character in a string?

Short answer: First non-repeating character in a string What interviewers test Hashing Single-pass logic Clean thinking Optimized solution public static char? FirstUniqueChar(string input)

Example code

{
var frequency = new Dictionary<char, int>();
foreach (var ch in input)
frequency[ch] = frequency.GetValueOrDefault(ch) + 1;
foreach (var ch in input)
if (frequency[ch] == 1)
return ch;
return null;
} Complexity Time O(n) Spac O(k) (unique chars) Why two loops? First: count Second: preserve order

Real-world example (ShopNest)

MNC rounds expect working code plus explanation. Walk through an example input from a ShopNest cart list, then discuss time and space.

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.