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”.
分析：元音字母有五个a,e,i,o,u，需要注意的是大写的也算，所以总共有十个字母。为了减小时间复杂度，在字符串反转这个代码中一般都会利用折半原理，两头往中间凑的方法。 Code：
public class Solution { public String reverseVowels(String s) { if("".equals(s)||s.length() ==1) { return s; } int length = s.length(); int leftIndex = 0; int rightIndex = length-1; char[] array = s.toCharArray(); do{ if(isVowel(array[leftIndex])) { if(!isVowel(array[rightIndex])) { rightIndex --; } if(isVowel(array[rightIndex])) { char tmp = array[leftIndex]; array[leftIndex] = array[rightIndex]; array[rightIndex] = tmp; leftIndex ++; rightIndex--; } }else { leftIndex++; } }while(leftIndex<rightIndex); return new String(array); } private boolean isVowel(char c) { return c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u' || c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U'; } }