public class Solution {    public string ReverseVowels(string s) {        var str = s.ToList();            var Vowels = new List
     
      ();            for (int i = 0; i < str.Count; i++)            {                var c = str[i];                if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u'                    || c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U')                {                    Vowels.Add(c);                }            }            if (Vowels.Count > 0)            {                Vowels.Reverse();                int j = 0;                for (int i = 0; i < str.Count; i++)                {                    var c = str[i];                                        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u'                    || c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U')                    {                        str[i] = Vowels[j];                        j++;                    }                }            }            StringBuilder sb = new StringBuilder();            foreach (var c in str)            {                sb.Append(c);            }            return sb.ToString();    }}