LeetCode Reverse Vowels of a String

原题链接在这里：https://leetcode.com/problems/reverse-vowels-of-a-string/

题目：

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".

Note:
The vowels does not include the letter "y".

题解：

典型的two pointers, 到了vowel时对调.

Note: 对调完要再移动双指针.

若用到Array.asList(arr), arr 这里要用Character型，而不是char型.

Time Complexity: O(s.length()). Space:O(s.length()), 因为建立了char array.

AC Java:

 class Solution {
     Character [] vowels = {'A', 'E', 'I', 'O', 'U', 'a', 'e', 'i', 'o', 'u'};

     public String reverseVowels(String s) {
         if(s == null || s.length() == 0){
             return s;
         }

         HashSet<Character> hs = new HashSet<Character>(Arrays.asList(vowels));

         int i = 0;
         int j = s.length()-1;
         char [] sArr = s.toCharArray();

         while(i<j){
             while(i<j && !hs.contains(sArr[i])){
                 i++;
             }

             while(i<j && !hs.contains(sArr[j])){
                 j--;
             }

             swap(sArr, i, j);
             i++;
             j--;
         }

         return new String(sArr);
     }

     private void swap(char [] arr, int i, int j){
         char temp = arr[i];
         arr[i] = arr[j];
         arr[j] = temp;
     }
 }