Reverse Vowels of a String(Easy)

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".

public class Solution {
public String reverseVowels(String s) {
    if(s == null || s.length()==0) return s;
    String vowels = "aeiouAEIOU";
    char[] chars = s.toCharArray();
    int start = 0;
    int end = s.length()-1;
    while(start<end){
        while(start<end && !vowels.contains(chars[start]+"")){
            start++;
        }

        while(start<end && !vowels.contains(chars[end]+"")){
            end--;
        }

        char temp = chars[start];
        chars[start] = chars[end];
        chars[end] = temp;

        start++;
        end--;
    }
    return new String(chars);
}