public class Solution {
    bool IsVowel(char c)
    {
        // or you could do
        // return "aAeEiIoOuU".Contains(c);
        return c == 'a' || c == 'A' || c == 'e' || c == 'E' || c == 'i' || c == 'I' || c == 'o' || c == 'O' || c == 'u' || c == 'U';
    }

    public string ReverseVowels(string s) {
        int i = 0;
        int j = s.Length - 1;
        char[] chars = s.ToCharArray();

        while (i < j)
        {
            if (!IsVowel(chars[i]))
            {
                i++;
            }
            else if (!IsVowel(chars[j]))
            {
                j--;
            }
            else if (IsVowel(chars[i]) && IsVowel(chars[j]))
            {
                char temp = chars[i];
                chars[i] = chars[j];
                chars[j] = temp;

                i++;
                j--;
            }
        }

        return new string(chars);
    }
}