345. reverse vowels of a string

class  {
    public String reverseVowels(String s) {
        if (s == null || s.length() < 2) {
            return s;
        }
        String vowels = "AaEeIiOoUu";
        Arrays.asList(vowels);
        char[] ch = s.toCharArray();
        int start = 0;
        int end = ch.length - 1;
        while (start < end) {
            while (start < end && !vowels.contains(ch[start] + "")) {
                start ++;
            }
            while (start < end && !vowels.contains(ch[end] + "")) {
                end --;
            }
            char temp = ch[start];
            ch[start] = ch[end];
            ch[end] = temp;
            start ++;
            end --;

        }
        return new String(ch);
    }
}