String - 345. Reverse Vowels of a String

class Solution {

    public static boolean[] vowels = new boolean[256];
    static{
        vowels['a'] = true;
        vowels['o'] = true;
        vowels['e'] = true;
        vowels['i'] = true;
        vowels['u'] = true;
        vowels['A'] = true;
        vowels['O'] = true;
        vowels['E'] = true;
        vowels['I'] = true;
        vowels['U'] = true;
    }

    public String reverseVowels(String s) {
        if(s == null || s.isEmpty()) return "";

        int i = 0, j = s.length() - 1;
        char[] str = s.toCharArray();
        
        while(i < j) {
            while(i < j && !vowels[str[i]]) i++;
            while(i < j && !vowels[str[j]]) j--;
            if(i < j) {
                char temp = str[i];
                str[i++] = str[j];
                str[j--] = temp;
            }
        }
        return String.valueOf(str);
    }
}