LeetCode刷题实战345：反转字符串中的元音字母
Given a string s, reverse only all the vowels in the string and return it.
The vowels are 'a', 'e', 'i', 'o', and 'u', and they can appear in both cases.
示例
示例 1：
输入："hello"
输出："holle"
示例 2：
输入："leetcode"
输出："leotcede"
解题
class Solution {
public final static HashSet<Character> vowels = new HashSet<>(
Arrays.asList('a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U')
);
public String reverseVowels(String s) {
if (s == null) return null;
int i = 0;
int j = s.length() - 1;
char[] result = new char[s.length()];
while (i <= j) {
char ci = s.charAt(i);
char cj = s.charAt(j);
if (!vowels.contains(ci)) {
result[i++] = ci;
} else if (!vowels.contains(cj)) {
result[j--] = cj;
} else {
result[i++] = cj;
result[j--] = ci;
}
}
return new String(result);
}
}