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”.

解题思路

双指针，一个往前移动，一个往后移动，找到元音字母时则交换。

实现代码

// Runtime: 12 ms
public class Solution {
    private static final String STR = "aeiouAEIOU";
    private static final Set<Character> VOWELS = new HashSet<Character>(STR.length());
    static {
        for (int i = 0; i < STR.length(); i++) {
            VOWELS.add(STR.charAt(i));
        }
    }

    public String reverseVowels(String s) {
        StringBuilder sb = new StringBuilder(s);
        int left = 0;
        int right = sb.length() - 1;
        while (left < right) {
            while (left < right && !isVowels(sb.charAt(left))) {
                ++left;
            }
            while (left < right && !isVowels(sb.charAt(right))) {
                --right;
            }

            char temp = sb.charAt(left);
            sb.setCharAt(left++, sb.charAt(right));
            sb.setCharAt(right--, temp);
        }
        return sb.toString();
    }

    private boolean isVowels(char ch) {
        return VOWELS.contains(ch);
    }
}