public class Solution {

    public string ReverseVowels(string s) {

        var str = s.ToList();

            var Vowels = new List<char>();

            for (int i = ; 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 > )

            {

                Vowels.Reverse();

                int j = ;

                for (int i = ; 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();

    }

}