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".

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        
        if (l1.val < l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
            
        }else{
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
        
    }
}