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".

Solution:
Just use two point from start and end of the string, respectively. 
BE CAREFUL ABOUT THE UPPER CASE!

Code(Java):

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public class Solution {     public String reverseVowels(String s) {         if (s == null || s.length() == 0) {             return "";         }                   char[] cs = s.toCharArray();                   int start = 0;         int end = cs.length - 1;                   while (start < end) {             while (start < end && !isVowel(cs[start])) {                 start++;             }                           while (start < end && !isVowel(cs[end])) {                 end--;             }                           if (start < end) {                 swap(cs, start, end);                 start++;                 end--;             }         }                   return new String(cs);     }           private boolean isVowel(char b) {         char c = Character.toLowerCase(b);         return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u';     }           private void swap(char[] cs, int start, int end) {         char temp = cs[start];         cs[start] = cs[end];         cs[end] = temp;     } }