345. Reverse Vowels of a String

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".

Note:
The vowels does not include the letter "y".

 答案：

    用首尾两个指针遍历，若两个都为元音，则交换，判断是否为元音可用一个set集合实现，看待判字符是否在set中。

class Solution {
public:
    string reverseVowels(string s) {
        if(s.length()==0){
            return s;
        }
        int i=0,j=s.length()-1;
        while(i<j){
            if(isvowel(s[i])&&isvowel(s[j])){
                swap(s[i],s[j]);
                i++;
                j--;
            }
            else if(isvowel(s[i])&&(!isvowel(s[j]))){
                j--;
            }
            else if(isvowel(s[j])&&(!isvowel(s[i]))){
                i++;
            }
            else{
                i++;
                j--;
            }
        }
        return s;
    }
    bool isvowel(char ch){
         set<char>vowels{'a','e','i','o','u','A','E','I','O','U'};
         if(vowels.count(ch)!=0){
             return true;
         }
         else{
             return false;
         }
    }
};