Leetcode 345: Reverse Vowels of a String

Write a function that takes a string as input and reverse only the vowels of a string.

Example 1:
Given s = "hello", return "holle".

Example 2:
Given s = "leetcode", return "leotcede".

Note:
The vowels does not include the letter "y".

 

public class Solution {
    public string ReverseVowels(string s) {
        if (s == null || s.Length < 2) return s;
        
        var vowels = new HashSet<char>();
        vowels.Add('a');
        vowels.Add('e');
        vowels.Add('i');
        vowels.Add('o');
        vowels.Add('u');
        vowels.Add('A');
        vowels.Add('E');
        vowels.Add('I');
        vowels.Add('O');
        vowels.Add('U');
        
        var arr = s.ToCharArray();
        
        int i = 0, j = s.Length - 1;
        
        while (i < j)
        {
            while (i < j && !vowels.Contains(arr[i]))
            {
                i++;
            }
            
            while (i < j && !vowels.Contains(arr[j]))
            {
                j--;
            }
            
            if (i < j)
            {
                var tmp = arr[i];
                arr[i] = arr[j];
                arr[j] = tmp;
            }
            
            i++;
            j--;
        }
        
        var sb = new StringBuilder();
        foreach (char c in arr)
        {
            sb.Append(c);
        }
        
        return sb.ToString();
    }
}