# Reverse Vowels of a String

[https://leetcode.com/problems/reverse-vowels-of-a-string/description/](https://leetcode.com/problems/reverse-vowels-of-a-string/description/)

> 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".

## Thoughts

题目要求把元音前后对调，因此需要两个指针分别指向需要对调的元素。

## Code

```
class Solution {
    private boolean isVowel(String s, int i) {
        String vowel = "aeiouAEIOU";
        return vowel.contains(s.charAt(i) + "");
    }
    public String reverseVowels(String s) {
        StringBuilder sb = new StringBuilder(s);
        int l = 0, r = s.length() - 1;
        while (l < r) {
            if (!isVowel(s, l)) {
                l++;
            }
            if (!isVowel(s, r)) {
                r--;
            }
            if (isVowel(s, l) && isVowel(s, r)) {
                sb.setCharAt(l, s.charAt(r));
                sb.setCharAt(r, s.charAt(l));
                l++;
                r--;
            }
        }
        return sb.toString();
    }
}
```

## Analysis

时间复杂度O(n).
