問題描述：

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.
s consist of printable ASCII characters
將原字符串中的元音字母倒序
注意審題，A與a均為元音，所以要考慮大小寫

思路：同樣是雙指針夾擊，當左側指針遇到元音字母時，停住并設立stopSign標記，否則前進；同理右側指針。若在某一時刻兩側指針均被stopSign標記，這時需要交換，交換后解除stopSign，并繼續靠近。

遇到的問題：沒審清題，未考慮元音字母存在的大小寫問題

代碼如下：
注：用stringbuilder方便交換字符串中的元素

public String reverseVowels(String s) {int left=0;int right=s.length()-1;char temp = ' ';StringBuilder ans = new StringBuilder(s);boolean stopSign_left = false;boolean stopSign_right = false;//when any pointer detects a vowel, stop, and do not move until the other pointer detects a vowelwhile(left<right){if (isVowel(ans.charAt(left))){stopSign_left = true;}else{left++;}if (isVowel(ans.charAt(right))){stopSign_right = true;}else{right--;}if ((stopSign_left)&&(stopSign_right)){temp = ans.charAt(left);ans.setCharAt(left, s.charAt(right));ans.setCharAt(right, temp);stopSign_left = false;stopSign_right = false;left++;right--;}}return ans.toString();}public boolean isVowel(char something){if ((something=='a')||(something=='e')||(something=='i')||(something=='o')||(something=='u')||(something=='A')||(something=='E')||(something=='I')||(something=='O')||(something=='U')){return true;}return false;} }

時間復雜度：O(n)

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