C# HasSet 1 Loop
\npublic class Solution {\n    public string GreatestLetter(string s) {\n        HashSet<char> set = new();\n        char c = char.MinValue;\n        for(int i 
```\npublic class Solution {\n    public string GreatestLetter(string s) {\n        HashSet<char> set = new();\n        char c = char.MinValue;\n        for(int i = 0; i < s.Length; i++)\n        {\n          set.Add(s[i]);\n          var lower = char.ToLower(s[i]);\n          var upper = char.ToUpper(s[i]);\n          if(set.Contains(lower) && set.Contains(upper))\n            c = (char)Math.Max(c, upper);\n        }\n        return c == char.MinValue ? string.Empty : $"{c}";\n    }\n}\n```
['C#']