A solution to the longest substring problem using HashSet

LongestSubstringProblemWithHashSet.cs

namespace DSA {
  public class LongestSubstringProblem {
    public static string HashSetSolution(string input) {
      if (string.IsNullOrEmpty(input))
        return string.Empty;

      var (candidateStart, candidateLength) = (0, 1);
      var current = new HashSet<char>();
      int start = 0;

      foreach (var character in input) {
        while (current.Contains(character)) {
          current.Remove(input[start++]);
        }
        // Mark the character as present in the current substring.
        current.Add(character);
        if (candidateLength < current.Count) {
          candidateStart = start;
          candidateLength = current.Count;
        }
      }
      return input.Substring(candidateStart, candidateLength);
    }
  }
}